Mamoru-kun ni Megami no Shukufuku o!

a.k.a. Mamoru was Blessed by a Goddess!.

Cute Shotaro Boy Mamoru Yoshimura has just joined Tokyo Beatrice University. Beatrice is The Force in this world that certain people can use. On his first day, he is asked out by Ayako Takasu, known on campus as the "Angel of Death". He discovers that, despite her reputation, she's really quite a nice person. Just Tsundere and has never opened up to anyone before. With the two of them an Official Couple, Mamoru joins the school's Absurdly Powerful Student Council to investigate incidents around the school. In the process, he tries to keep Ayako's temperament under control and to know her better. The younger sister of one of Ayako's rivals joins the Council later due to her admiration of Ayako.

This 2006 anime series by ZEXCS ran for 24 episodes and was based on a series of Light Novels. Compare Kanokon, which has a similar Love Triangle set up, but is Hotter and Sexier.

The anime can be legally viewed at Crunchyroll here.

Tropes used in Mamoru-kun ni Megami no Shukufuku o! include:

Absurdly Powerful Student Council -- To emphasize the point, the Student Council room is closer to an evil overlord's lair than anything else. They even use their humongous wall of TV screens to spice up conversation.
- For the amount of resources that seem available to them, for some reason they seem to use it mostly to spy on Mamoru and Ayako, which often causes Ayako to beat them down. Occasionally they'll also use it for good purposes.
Airplane of Love
Anguished Declaration of Love -- Emelenzia has one in episode 19 for Mamoru and says it while Ayako is present
Anime Hair -- Shione, whose hair has Princess Curls and, at times, goes downright Seussian. This is lampshaded; in one episode where she makes her hair straight, nobody recognizes her.
Anti-Magic -- Horacle, a gem that naturally absorbs the power of Beatrice in the area. Ayako being Brought Down to Normal by one forms the last arc of the series.
Battle Aura -- Ayako and several others.
Beach Episode -- Episode 17
Berserk Button -- Anytime Ayako catches the student council spying on her and Mamoru or cracking jokes about their relationship.
Big Brother Complex -- Shione, again.
Big Fancy House -- Once again, Ayako.
Breather Episode -- Episode 15, after the intense and dramatic events in episodes 13 and 14.
Brother-Sister Incest -- YMMV, but Itsumi, Mamoru's sister, seems a little disturbed after seeing him kiss Ayako during the play, and then later how close and friendly they are to each other. The look she has during those scenes almost seem to be of jealousy, as if Itsumi wants her brother to be a bit closer to her...
Clingy Jealous Girl -- Ayako tends to display this if Mamoru is spending too much time with another girl, particularly Emelenzia.
Cloudcuckoolander -- Yagi of the Student Council. He has a tendency to speak in Gratuitous English and borders on being a Hard Gay.
Cute Shotaro Boy -- Mamoru
Defeat Means Friendship -- What happens to Emelenzia after seeing Mamoru and Ayako fighting together.
Dropped Glasses -- When Abiko loses his glasses, he was apparently a real looker beneath them. This was his manifestiation of Beatrice power.
Dynamic Entry -- Combined with a through-the-roof Super Window Jump and ending with Ayako's foot on Yagi's head.
Exposed to the Elements -- As Emelenzia found out, lingerie and winter weather don't mix.
Flying Car -- Not only can it fly, but Mamoru can drive it! (does he even have a license?)
Forgotten First Meeting: It gradually becomes clear that the Beatrice-user who saved Mamoru during a disaster when he was small was Ayako's uncle - and what's more, she was there too. Neither one of them remembers at first due to having been quite young and traumatized to boot, but that event was the reason why Mamoru decided to become a Beatrice-user himself, which led to him joining the Beatrice Academy, where he, of course, met Ayako again...
Gratuitous German -- Emelenzia.
Heel Face Turn -- Emelenzia and her two lackeys. In at least one instance, they help bring him to Ayako.
Heroic BSOD -- Ayako and Mamoru suffer these from time to time.
Hot Springs Episode -- Maya and Shione turn delivering a bottle of shampoo into Serious Business.
Huge Schoolgirl -- Ayako
- Break the Cutie in that same episode.
Instant Fanclub -- Emelenzia gets one shortly after joining.
I Just Want My Beloved to Be Happy -- Emelenzia, despite having feelings for Mamoru, often helps him get back with Ayako. She often claims that she's doing it so that she can steal him away fair and square.
Love Triangle -- Type 4 Triang Relations. Emelenzia is A to Mamoru's B and Ayako's C. It doesn't stop Emelenzia from trying however.
Malaproper -- While she speaks Japanese perfectly, Emelenzia doesn't always get it right.
Megaton Punch -- Megaton Kick, Megaton Throwing-a-Table-at-People...all from Ayako.
New Transfer Student -- Emelenzia.
Out-of-Clothes Experience -- When Mamoru enters into Ayako's soul to free her from problems (twice during the series), they're always without clothing, though Barbie Doll Anatomy is in full effect.
Parental Abandonment -- As explained by Ayako in the third episode.
The Power of Love -- Ayako explains it herself -- when Mamoru's with her, she's invincible.
Really Seven Hundred Years Old -- Marie, who can change her age from a young child to a grown woman in an instant.
School Newspaper Newshound -- Mitsuki, who's never seen without her camera.
School Play -- Sleeping Beauty, used for Tonight Someone Kisses.
Snow Means Love -- There's a reason an episode or two is spent at the ski slopes.
Stalker with a Crush -- Tsuneo Abiko, Ayako's crazed stalker with Nerd Glasses, Idiot Hair, and way too much free time.
Tall, Dark and Bishoujo -- Ayako
Tiny Guy, Huge Girl -- Our Official Couple, Mamoru and Ayako, of course. Taken to ridiculous levels early on in the series during a car chase when she carries him like a baby.
Tsundere -- Ayako (duh!). Type A all the way. Has her dere moments in Mamaru's company (now if everyone would only leave them alone!)
Xanatos Gambit -- What drives the non-school plot portions of the series. Maria seems to be against the research with beatrice that Johan and Misaki are doing, and tries to do things to stop it. While she failed initially, they learn much more about the beatrice after seeing Mamoru and Ayako in action at the end of the series, and they ultimately begin to research using beatrice for good rather than selfish purposes.