The Girl in the Library: Until The Pure You Falls From Grace

The Girl in the Library: Until the Pure You Falls From Grace (Toshoshitsu No Kanojo ~Seiso Na Kimi Ga Ochiru Made~) is a short manga series written by Arare Ame and adapted into a four-episode OVA by Pink Pineapple faithfully using the first four chapters. While the manga was originally meant to end at chapter four, a fifth chapter was eventually made along with a sixth chapter which concluded the story. An OVA based off the fifth chapter was released on June 2023 with a final OVA announced for September 2023.

The story begins when a high school student, Yukiha Serizawa, while working as a library assistant with her crush, Aida, defends the janitor, Masami Kitou, after he's accused of breaking a vase. Masami, in turn rapes her while she's drugged while recording it and then blackmails her into being his sex slave. However, Masmai proves to be really good at sex which Yukiha begins to enjoy and eventually becomes his sex slave of her own free will.

"Yukiha: "This number... No... If I answer this call, there will be no going back for me... ever again..." (heads to Masami's house) Masami: "You know what going in my place means, right?""
 * Abusive Parents: Both Yukiha and Masami will become this. Masmai plans to rape his child if it's a girl or abort it if it's a boy, and Yukiha's become too much of a slut to care.
 * All for Nothing: The entirety of episode/chapter 2 is this, with Masami refusing to delete the sex video he made even after Yukiha humiliated herself.
 * Takamine is forced to play a "game" to free herself and Yukiha by making Masami orgasm before she does. Though she is successful, she is shocked when she figures out too late that Yukiha was with Masami of her own free will.
 * Babies Ever After: Yukiha ends up pregnant with Masami's child when she become his sex slave willingly. Both are hoping for a girl so Masami can turn his own daughter into his sex slave as well.
 * Bad Date: Aida and Yukiha were boyfriend and girlfriend for less than two days with their first date being the last. Why? Well...
 * 1) Masami finds out where Yukiha lives and rapes her, ripping her new clothes in the process on what was meant to be the start of the date.
 * 2) Yukiha is unable to concentrate on the movie Aida took her to because of the umm... aftermath of the rape.
 * 3) Masami forces Yukiha into a public restroom where she is once again raped only she begins to enjoy it.
 * 4) Aida finds out what the janitor had been doing to Yukiha and vows to protect her, but when he has sex with her at her request, he doesn't even measure up to Masami's prowess.
 * 5) Yukiha is left begging for an orgasm when she sees a number on the phone.  Knowing full well she'll ruin her life if she answers she goes to Masami's house of her own free will.
 * 6) With Yukiha now a slut, Masami has sex with her on the condition Aida is told that Yukiha is Masami's woman now.  She agrees and the two have sex on Aida's phone, breaking his heart.  Masami and Yukiha have sex until morning which leaves Yukiha barely conscious, twitching, and pregnant.
 * 7) In the end, their date ends with two lives ruined, with Aida being driven insane and Yukiha expelled from school, her reputation among her peers destroyed and disowned by her parents and yet had become so evil and slutty, she doesn't even care.
 * The Bad Guy Wins: Masami not only gets away with raping Yukiha over and over, but she becomes his sex slave of her own free will.
 * Be Careful What You Wish For: Takamine wanted to find out what happened to her student, Yukiha. When she finds out, it seals her fate as another sex slave.
 * Bigger Is Better in Bed: Sadly Masami is this trope which played a role in Yukiha becoming his sex slave willingly.
 * Blackmail: Masami threatens to release a sex tape he made when he raped Yukiha if she doesn't agree to be his sex slave.
 * Break the Cutie: Yukiha, a kind-hearted girl is repeatedly raped by the janitor and becomes so corrupted she becomes almost as bad a person as he is.
 * Also Aida, a kind-hearted boy suffers an immense mental breakdown after the girl he loves dumps him for her own rapist.
 * Buxom Is Better: Masami believes Yukiha's boobs are F or G cup when he first sees them. Naturally he enjoys fondling them before raping her and having sex with her when she becomes his sex slave willingly.
 * Character Development: Yukiha develops the most out of all the characters but not in a good way. Yukiha starts off the story as a sweet, innocent schoolgirl that fell for a kind hearted boy but unfortunately became a sex slave to the evil Masami.  However near the end of the 3rd chapter/episode, she finds herself desperately needing an orgasm when the boy she loved failed to make her experience it.  Driven to madness, she runs to Masami and is left begging him to have sex with her.  She ends up making Aida watch as she and Masami have sex, breaking his heart and driving him insane and cuts all ties with her school, job and even her own parents to be Masami's sex slave and ends up pregnant with his child and doesn't care if he turns their daughter into a sex slave too or abort their son as long as Masami has sex with her.  By the fifth chapter, she ends up being just as evil as Masami by helping him rape her teacher, Takamine, the only person left that actually cared about her.  She doesn't even have any love for Aida by this point and forgets he even existed as all she lives for is being pleased by Masami.
 * Child by Rape: Averted, Yukiha becomes pregnant after she becomes Masami's sex slave willingly.
 * Cool Motive, Still A Crime: While Masami raping her and making her his sex slave was the catalyst, Yukiha made the choice to go to Masami's house of her own free will for an orgasm. She also agreed to break Aida's heart and drive him insane by making him watch her and Masami have sex. Masami himself even gave her the chance to escape him once and for all by letting her choose between him and Aida. In the end the one that made the once pure-hearted Yukiha fall from grace was Yukiha herself.
 * Defiled Forever: Masami took Yukiha's virginity when he raped her. Because of that she felt unworthy of Aida's love.
 * Deliver Us From Evil: Inverted, the night Yukiha is impregnated by Masami is the night she becomes as evil as Masami.
 * Derailing Love Interests: Played darkly here, Aida is a very friendly person but his poor sexual performance made Yukiha dump him for Masami.
 * Despair Event Horizon: Aida after Yukiha not only has sex with Masami while he watches on his phone but mocks his poor sexual performance. Even after six months, he's still not only traumatized, but became a shut-in with garbage piling up in his room.
 * Takamine goes through this when she realizes Yukiha is Masami's sex slave by choice, and the shock of betrayal she goes through makes her lose the will to resist Masami and the vice principal eventually becoming another sex slave.
 * Did Not Get the Girl: Aida not only doesn't get to be with Yukiha, but she openly mocks his poor performance on his phone while he's forced to watch her have sex with her own rapist willingly.
 * Did Not Think This Through: Averted with Yukiha in the third chapter. She knew damn well she was going to ruin her life if she went to Masami's house for sex but she did it anyway.  Even Masami gave her a chance to escape this.


 * Dirty Old Man: Masami is possibly in his fifties but he forced Yukiha, a high school girl, to be his sex slave and eventually got her pregnant and even hopes to turn his future daughter into a sex slave as well. He even calls himself a dirty old man when he first rapes Yukiha.
 * Disproportionate Retribution: For his failure to make Yukiha experience an orgasm, Aida is forced to watch Yukiha have sex with Masami of her own free will while being mocked for his poor stamina and small penis. Aida's heart is not only broken but he suffers a complete mental breakdown he's still suffering from six months later.
 * Downer Ending: Chapter/episode 4 which was meant to wrap up the story ends with Aida completely heartbroken, Yukiha disowned by her parents and expelled from school and pregnant with Masami's child, whom he plans to rape if it's a girl and abort if it's a boy.
 * Chapter 6 which concludes the story ends with Takamine accepting that Yukiha is beyond redemption and in her despair no longer able to resist Masami and the vice principal and ends up in a porno with Yukiha, presumably destroying her career.
 * Ephebophile: Masami makes the high schooler Yukiha his sex slave. He ends up getting her pregnant by the 4th chapter/episode.
 * Et Tu, Brute?: Takamine's reaction to being raped by the vice principal and Yukiha saying she's a sex slave of her own free will. The shock makes her unable to resist fighting Masami and the vice principal and she becomes another sex slave.
 * Even Evil Has Standards: When Yukiha went to Masami's house of her own free will, Masami himself warned her once she enters his house there'd be no going back to her old life.
 * Yukiha was perfectly okay with breaking Aida's heart and mocking his performance, but for some reason she begged him not to look at her orgasm face.
 * Everyone Calls Him "Barkeep": Masami is often called the Janitor or Mr. Janitor by Yukiha and Aida.
 * Evil All Along: The vice principal always hated Takamine and joins Masami in raping her.
 * Exact Words: In chapter/Episode 2, Yukiha begs Masami to stop making her his sex slave. Masami agrees to give some thought about deleting the sex video he made if she humiliated herself at school wearing bondage straps and a vibrator jammed into her all day.  When she's done, he begins fondling her again.  When she tells him that he wasn't keeping his promise, he tells her he said he'd only give it some thought.  Yukiha then realizes Masami had no intention of deleting that video.
 * Face Heel Turn: Yukiha was corrupted by Masami's sexual prowess and became just as evil as he was by chapter 5.
 * Fair Play Villain: Masami removed Takamine's restraints during their "game" to give her a fair edge.
 * Fan Disservice: Masami is a well-endowed sex god. But he's also an ugly bastard with hairy legs and is possibly in his 50s and is also a vile rapist.
 * Yukiha going Naked Apron while pregnant. Doesn't stop Masami from having sex with her.
 * Basically any sex scene in general.
 * The Farmer and the Viper: This is basically the hentai form of this trope. Masami already had a shady and perverted reputation long before the story began, being accused at leering at the schoolgirls.  So when Yukiha pointed out that the vase he supposedly broken was already cracked and damaged, Masami repays the favor by raping her and blackmailing her into being his sex slave.
 * Faux Affably Evil: By chapter 5, Yukiha maintains a sweet and almost innocent personality, but assists Masami in raping her teacher, Takamine, showing she has become just as evil as he is.
 * Forced to Watch: Masami and Yukiha have sex while making Aida watch on his phone. While technically he could turn his phone off, seeing the love of his life mock and belittle him while having sex with her own rapist willingly leaves him too broken hearted and shocked to put the phone down.  By the time he finally does stop watching, he's already suffered a mental breakdown.
 * For Want of a Nail: Everything that happened in this series from Yukiha being a sex slave, to becoming as evil as her rapist, to her life being utterly destroyed, to even her teacher becoming a sex slave as well, and her boyfriend being driven insane, wouldn't have happened if she didn't tell Takamine that the vase Masami supposedly broke was already cracked and damaged.
 * Future Loser: Yukiha had a promising life ahead her her, a loving boyfriend, respected at school, a nice job, a stable home life, but she threw all that away to be a janitor's sex slave. Now she burned her bridges with her school, Aida and even her own parents have disowned her for her pregnancy.  She's still a teen but she's already destroyed her whole life and she doesn't even care.
 * Gainaxing: Yukiha when she has sex with Masami.
 * Go Mad From The Revelation: Aida's heart is not only broken when Yukiha chooses her own rapist, but being forced to watch her have sex with Masami on his phone makes Aida suffer a complete mental breakdown he's still suffering from six months later.
 * Yukiha has only ever had sex with Masami and even lost her virginity to him when he raped her, so when Aida fails to pleasure her, she tries in vain to reach orgasm by masturbation and realizes Masami is now the only one who can pleasure her.
 * Good People Have Good Sex: Unfortunately averted and inverted. Though Aida loves Yukiha, his poor performance in bed makes Yukiha dump him for her own rapist, becoming Evil People have Good Sex.
 * Happiness in Slavery: Yukiha becomes too dependent on Masami to sexually pleasure her that she becomes his sex slave willingly and even calls herself an onahole (a Japanese sex toy).
 * Hate Sink: Masami is a pervert that drugged and raped Yukiha shortly after she saved him from being fired over a vase and then recorded the rape and made her a sex slave by threatening to leak the video online.  He then humiliated Yukiha by making her put bondage straps over her clothes and jam a vibrator in her which he would activate when she was called on by her teachers.  By the time Yukiha becomes his sex slave of her own free will and Masami gets her pregnant, he makes it clear he wants his own unborn child to be another sex slave if it's a girl or abort if it's a boy.
 * Yukiha herself becomes a hate sink when she chooses to be Masami's sex slave willingly and then broke Aida's heart by making him watch her and Masami have sex and cause him to suffer a mental breakdown. Once she's impregnated by Masami she also hopes to give birth to a daughter just to please Masami's desire.  By the the fifth chapter, she assists Masami in raping her teacher and became just as evil as he is and destroying any sympathy she might have gotten as a rape victim.
 * The Hedonist: Yukiha pretty much states she now lives to be pleasured by Masami by chapter 6 and doesn't give a damn about the fact that she basically ruined her life to do so because in order to be pleasured by Masami, Yukiha ended up in porn that went viral, got expelled from school, disowned by her parents for early pregnancy, and drove the boy who loved her insane. Masami is now the only bridge left she hasn't burned so, if he were to die or he kicks her out, she'd likely end up homeless.
 * Humiliation Conga: In episode/chapter 2, after raping Yukiha, Masami tells Yukiha he'd consider deleting the sex video on his phone if she'd place bondage straps under her school clothes with a vibrator jammed in her for the day. Whenever the teachers would call for her, Masami, watching through hidden cameras would activate the vibrator.  When Aida confessed his love for Yukiha, Masami activated the vibrator before she could confess her love for him as well.  In the end, he refused to delete the video.
 * The Immodest Orgasm: Seriously how did they not get caught with how often Yukiha screams during sex?
 * Important Haircut: Inverted with Yukiha. She starts the story with shoulder length hair and a kindhearted nature.  Six months after becoming Masami's sex slave willingly, her hair goes to her waist and she ends up being as evil as he is, showing no concern for the wellbeing of her unborn baby and helping Masami rape her teacher, made her teacher end up in a porno that likely ruined her teaching career, and pretty much forgetting the kindhearted boy who loved her even existed.
 * Also inverted with Aida. Once his heart is broken by Yukiha and driven insane, his hair grows to shoulder length.
 * Irony: For the first 3 chapters, Yukiha is frequently raped by Masami, but when Aida fails to make her experience an orgasm, she has to beg Masami for sex in the fourth chapter.
 * I Gave My Word: Masami releases Yukiha from being a sex slave after losing to Takamine.
 * I Have No Daughter: Yukiha's parents disowned her when she ran off with Masami and got pregnant. Considering she has since become just as evil as Masami and only cares about the pleasure she receives from him, they were right to disown her.
 * I Showed Her What A Real Man Is: Masami mocks Aida when Yukiha becomes his sex slave willingly.
 * I Want My Beloved to Be Happy: Calling Yukiha Masami's beloved is a huge stretch but he said he'd leave her alone if she decided to be with Aida when she was torn between the two.
 * I Was Quite a Looker: Backstories in the manga suggest Masami was quite handsome when he was much younger.
 * I Will Protect Her: Aida vows to protect Yukiha from Masami. Unfortunately his poor performance drives Yukiha into Masami's house.
 * Takamine tries to save Yukiha from being Masami's sex slave by playing his "game." Like with Aida, it goes poorly.
 * Idiot Ball: Takamine carries it heavily in the fifth and six chapters. Six months after Yukiha disappeared, she apparently never knew anything about the circumstances behind Yukiha's expulsion and being disowned by her parents despite Yukiha being in her class and the rumors about Yukiha circulating and even porn of her showing up on the internet and continued to prod despite constant warnings.  Once she's told where to find Yukiha she's ordered to go alone without the police and ends up in Masami's clutches.  Even after Yukiha helps Masami rape her teacher, Takamine still tries to save Yukiha, only to be shocked to see Yukiha run into Masami's arms and kiss him. Yukiha finally has to spell it out for Takamine that she's Masami's sex slave by choice.  Also Masami removed all her restraints during the "game" so Takamine with some quick thinking could have easily made her escape and go to the police.
 * It Amused Me: The real goal Masami had in mind when Yukiha humiliated herself in chapter/Episode 2.
 * "It's Not Rape If You Enjoyed It": Yukiha starts to enjoy sex with her rapist and dumps Aida for failing to pleasure her.
 * Karma Houdini: Masami gets away with his actions and gains Yukiha as a sex slave and even gets money making porn with her.
 * Subverted with Yukiha. While she isn't actually punished for driving Aida insane or helping her teacher get raped, she ruined her whole life when she became Masami's sex slave willingly and porn of her showed up on the internet, which gets her expelled from school, her reputation among the students shattered, and even gets her disowned by her parents.  Masami is now the only bridge she hasn't burned so if he dies or she burns that too, she's screwed.
 * Love Hotel: Aida and Yukiha go to one.
 * Making Love in All the Wrong Places: More like being raped in all the wrong places. Masami has raped Yukiha in the school library and a public restroom and didn't stop even when another person entered.
 * May-December Romance: Yukiha is a high school girl while Masami is a man in his 50s. However it is no romance.
 * More Than Mind Control: Even after Yukiha helps Masami rape Takime, the teacher is implied to believe that Yukiha is acting against her will. Once Yukiha is freed thanks to Takamine's efforts, Yukiha's first act is to kiss Masami and then spell it out for her former teacher that she's Masami's sex slave of her own free will.  Takamine is forced to accept Yukiha is a lost cause because of it and breaks down becoming another sex slave.
 * Morton's Fork: In chapter/episode 4, Yukiha is torn between Aida and Masami.  Masami offers her a choice: he'd actually back off and stop raping Yukiha if she decides to go with Aida, but she'd live a boring life with a boy with almost no sex drive but at least be with some someone who loves her.  However if she'd pick Masami, she live her life as a sex slave with a man who has no love for her but is a well-endowed sex god.
 * The gender of Masami and Yukiha's unborn baby is also this. If it's a boy, it gets aborted but if it's a girl, it becomes another sex slave for Masami.
 * Naked Apron: Yukiha wears it after being becoming Masami's sex slave willingly. Also she's pregnant while wearing it.
 * Netorare Genre: It begins near the end of episode/chapter 3 and goes into full force in episode/chapter 4 when Yukiha has sex with Masami of her own free will while openly mocking Aida as he watches.
 * Nice Guy: Aida, often helping Yukiha at the library and vowed to protect Yukiha from Masami.
 * No Pregger Sex: Very averted. Masami not only has sex with a six-month pregnant Yukiha, but even records it saying he'd be paid a lot of money for a sex video with a pregnant high schooler.  He along with the vice principal continue having sex with her through chapters 5 and 6.
 * Official Couple: Yukiha and Masami, unfortunately.
 * Offing the Offspring: Masami plans to abort his and Yukiha's unborn child if it's a boy.
 * Oh Crap: Yukiha does this twice. In chapter/episode 2, when she realizes Masami never planned to delete the sex tape even after she humiliated herself and again in chapter/episode 3 when she learns Masami knows where she lives.
 * Parental Incest: Masami plans to have sex with his own possible future daughter who hasn't even been born yet. Yukiha is perfectly okay with this.
 * Porn with Plot: Well if you can call it a plot.
 * Psychopathic Womanchild: Ultimately became the once kindhearted Yukiha's personality after six months of living with Masami. When Takamine arrives at Masami's house, she finds Yukiha completely nude aside from parts of an outfit and pregnant.  Yukiha mistakes the door opening for Masami and begs for sex saying she took care of the house while he was gone almost like a child expecting a reward.  When Takamine tries to "save" Yukiha, Masami tasers the teacher while Yukiha looks at her and smiles as she loses consciousness.  While Masami is raping Takamine, Yukiha grabs a vibrator and uses it to help rape Takamine.  When Takamine begs Yukiha to turn it off, Yukiha says that "onaholes" are supposed to be quiet when the master has sex with them and sets the vibrator to its max setting to "teach Takamine a lesson for ordering her around."
 * Pyrrhic Victory: In chapter 6, Masami decides to play a "game" with Takamine. If she can make him orgasm before she does, he'd let both her and Yukiha free.  Takamine is able to win the "game," guaranteeing Yukiha's freedom, only for Yukiha to point out she became Masami's sex slave by choice.  In her despair, Takamine loses her will to resist Masami and the vice principal and becomes another sex slave.
 * Reality Ensues: This is how Masami threatening to leak the sex video he made when he raped Yukiha worked. Even though that could also incriminate him and get him arrested, that video on the internet would have also been very humiliating for Yukiha.  And worse, it's extremely difficult to get something off the internet and most of the time, impossible so that video would end up haunting her for life.
 * Yukiha's first sexual experience was with Masami, a well-endowed sex god while Aida was a virgin with no experience so when Aida has sex with her, she's left disappointed by his poor performance and small penis leaving her frustrated and desperate. It does not justify what she does afterwards.
 * If you were to make the argument that Aida could have put his phone down when Yukiha and Masami post themselves having sex, consider this. The love of his life is not only cheating on him, but with her own rapist while she mocks and belittles him.  He's mentally unable to put his phone down due to shock and a broken heart.  By the time he finally was able to, he was driven insane.
 * Reasonable Authority Figure: The teacher Takamine is the only one who shows concern when both Yukiha and Aida disappeared from school. It ends up making her a target for Masami.
 * Redemption Rejection: Masami actually gives Yukiha a chance to escape him once and for all if she chose Aida over him. She however becomes such a slut she chooses to be a sex slave to Masami than be with Aida who loved her.
 * Sex Face Turn: Inverted, Yukiha becomes corrupted and evil after she becomes Masami's sex slave.
 * Sex God: Masami is an ugly bastard and a vile rapist, but it can't be denied he is great at sex. When Yukiha become his sex slave willingly, they have sex until morning, at which point while Masami claims to finally be worn out, Yukiha is barely conscious.  She describes sex with him as heaven.  Even the act of sucking her nipples while licking them made Yukiha have a wild orgasm.  Even when she performed fellatio on him, she came before he did.
 * Sex Is Evil: Sex is never, ever presented in a positive light in this hentai. First off Yukiha loses her virginity when Masami first rapes her and then makes her into a sex slave and humiliating her and torturing her with it.  When she finally has sex with Aida, he leaves her so unsatisfied she becomes Masami's sex slave of her own free will.  Their first consensual sex scene isn't any better than the rape when the two force Aida to watch on his phone, breaking his heart and driving him to insanity.  Once Yukiha is impregnated by Masami, she rightfully gets expelled from school and disowned by her parents leaving Masami as the only bridge she has left not that she cares since she becomes as evil as Masami is with both hoping for a daughter to be made into another sex slave.  Finally Takamine is raped nonstop, betrayed by both the vice principal and by Yukiha who she was trying to save and is left so broken she becomes another sex slave as well and ends up in a porno, likely destroying her teaching career.
 * Sex Slave: Yukiha was originally this due to blackmail, but as she becomes more slutty and corrupted, she becomes this of her own free will.
 * Takamine suffers this at the end of chapter 6.
 * Single Woman Seeks Good Man: Yukiha originally fell in love with Aida because of his kind heart. However, his poor performance made her dump him to be Masami's sex slave instead.
 * Slipping a Mickey: How Masami raped Yukiha to begin with.
 * Slut Shaming: The sex tape made by Masami gets leaked out causing Yukiha to be expelled from school and disowned by her parents for early pregnancy. Justified since she became an evil woman when she become Masami's sex slave of her own free will and is okay with her unborn child being raped or aborted and aided in her teacher being raped.
 * Speed Sex: Aida though it's understandable since he was likely a virgin with no experience. He attempted another round suggested by two condoms in the trashcan but his energy ran out and the poor experience drove Yukiha into the arms of her own rapist.
 * Stay in the Kitchen: Six months after running off to be Masami's willing sex slave, she's completely disappeared from everyone she knew and is last seen cooking dinner for her and Masami, hinting she's never left his house, even when he's gone.
 * Teen Pregnancy: Masami was determined to get Yukiha, a senior high school student pregnant after raping her and making her his sex slave. He succeeds by the fourth chapter when she become his sex slave willingly.
 * Teeny Weenie: Yukiha mocks Aida by saying he has the dick of a child while praising Masami's while having sex with him while Aida watches. This breaks Aida's heart and drives him to madness.
 * Took a Level in Jerkass: Yukiha after she becomes Masami's sex slave willingly. She's implied to be perfectly okay with her possible future daughter being raped, or her possible son being aborted.  She also mocked Aida's poor performance while he listens.  In chapter 5, she even helps Masami with raping Takamine, becoming just as evil as he is.  By chapter 6, she barely even remembers Aida saying she only exists to be pleasured by Masami.
 * Undying Loyalty: By chapter 6, Yukiha is completely devoted to Masami that when Takamine is able to grant her freedom, Yukiha's immediate reaction is to kiss Masami in front of Takamine. She also has no more love for Aida and barely acknowledges his existence.
 * Ungrateful Bastard: Masami's reaction to Yukiha saving him from being fired is to rape her.
 * Yukiha shows no gratitude to Takamine freeing her from being a sex slave and kisses Masami once she's freed.
 * Unholy Matrimony: Masami and Yukiha become somewhat of a couple when she becomes his sex slave willingly. She also ends up being as evil as he is.  They're also the official couple as shown above.
 * Used to Be a Sweet Kid: See Character Development for Yukiha.
 * Victim Falls For Rapist: It's not so much Yuikiha falls in love with Masami, she just becomes a slut that just wants him for his sexual prowess. She did however offer her heart to him in return for sex.  By chapter 6, she's completely devoted to him.
 * Villain Has A Point: In chapter/Episode 4, Masami offers to have sex with Yukiha if she tells Aida she offered her body and heart to him. When Yukiha shows some reluctance to do that, Masmai points out she left Aida to have sex with him.
 * Villainous Friendship: Masami and the vice principal. Masami even lets him have sex with Yukiha.
 * Was It Really Worth It?: As pointed out in the tropes, Yukiha basically destroyed her life to become Masami's sex slave and burned every bridge she ever had. Since Masami is all she has left, it's a matter of when, not if, Yukiha's poor choices come to bite her in the ass.  And when it does, one may ask her if the sex was really worth it.
 * Women Prefer Strong Men: Masami is definitely stronger than Aida and has an infinitely stronger sex drive. This is what makes Yukiha choose him over Aida.  By chapter 6, Masami is redrawn to show abs and large biceps.