Undertale/Good Route

The good route of Undertale, most commonly referred to as the Pacifist route, is the one in which the player resolves each monster encounter without taking a single life, thus winning the trust of the Underground and, ultimately, allowing them to migrate freely back to the surface to start a new life. Along the way, you end up hanging out with each of the main monster characters, getting to know their personalities and backstories.

WARNING! There are unmarked Spoilers ahead. Beware.

Tropes appearing specifically in Undertale's Pacifist route include:

Abandoned Laboratory: The True Lab, the last main area in a Pacifist run is an old lab of Alphys' that has fallen into disrepair. It isn’t entirely abandoned, but it looks the part, and is filled with all the horrors you’d expect from a place like it.
Bitch in Sheep's Clothing: Yes, it is possible to complete the Pacifist route while generally being a dick to everyone; as long as you don't kill anyone (or leave them for dead, in the case of Undyne), you can be as horrible as you like. Surprisingly, Sans the Skeleton won't call you out on your douchebaggery.
- That being said, there are two minor characters that will call you out in the Pacifist ending sequence.
  - If you simply stood around without doing anything while Monster Kid was hanging for dear life from a bridge, they'll remember how you just sat around and watched them almost die. If you try talking to them, they'll stammer for a bit, before quickly telling you not to talk to them, and remain mute afterward.
  - If you had previously eaten the Snowman Piece in front of the talking snowman, he'll have some harsh words for you:

"Ah, the barrier's open...? You know I cannot move. Why are you telling me? To mock me? Everyone may think you are a good person, but this snowman knows the truth. Someday, your friends will realize your heart is as cold as my butt."

Amazing Technicolor Battlefield: The first phase of the True Final Boss of the Pacifist route has the background as a changing wave of colors while the boss moves around it. The second phase has the boss's wings with moving colors as well.
Apocalyptic Log: Two of them, both in the same hidden location. The True Laboratory has the written logs from Alphys and the audio tapes from before the deaths of Asgore's children.
Bleak Level: The True Lab. Its only residents are Alphys and a mysterious new class of enemies, the color scheme is dull and dark, and it provides exposition about the Alphys's first experiments, Flowey’s origins, and the past of the Dreemurr family, all of which are… not lighthearted.
Body Horror: The Amalgamates in the True Lab. Up to dozens of monsters, injected with Determination, now sharing a same body in an attempt to hold some physicality.
Cringe Comedy: The date with Alphys on the Pacifist route. Between Alphys's complete lack of social skills, and the fact that she's obviously pining for Undyne, it's awkward from the word "go". But then you start to roleplay an interaction between Alphys and Undyne, and things get even worse (or better, since that's when the scene goes from amusing to hilarious). And then Undyne overhears you.
Driven to Suicide: Asriel's sibling, the Fallen Child, ended their life to enact a grand plot to kill the humans and reclaim the surface world for the monsters. But what you may not know is that they tried to kill themselves once before. If you visit Asriel in the Ruins at the end of the Pacifist route, he talks about what led the first human to climb Mt. Ebott, the one-way entrance into the Underground. The legend goes that no humans who climb the mountain ever return, and the child wanted so badly to disappear... We can only guess at what could drive someone to such desperate measures.
Freaky Miniboss Squad: The Amalgamates.
Earn Your Happy Ending: If you spared every enemy and got to the final room with no kills, not only did you not increase your LV (meaning you beat the game on the starting 20 HP), you still don't get the good ending unless you've befriended everybody. Then after that, you go through the True Lab, and found out the secrets of the hidden lab, as well as the truth of Flowey's existence, followed by another boss battle.
Epiphany Therapy: Subverted with Alphys in the Pacifist run. Although she confesses to Undyne that she feels like a fraud and has lied about anime, and Undyne tells her she loves Alphys Just the Way You Are and enacts Tough Love by making her run with Papyrus, Alphys still doesn't feel great. You find a note before entering the True Lab that reveals she might not come out, which means that by following her, you could potentially interrupt her off-screen suicide because she protects you from the enemies within the True Labs. This is Truth in Television: someone suffering social anxiety and depression won't just get better and may even feel lower after a high of hearing someone cares about them.
Four-Temperament Ensemble: The four monsters you befriend in the Pacifist route. Papyrus (sanguine), Undyne (choleric), Alphys (melancholic) and Sans (phlegmatic).
Have We Met?: In the True Pacifist finale, Sans and Toriel, two characters who had never met in person, find each others' voices familiar and realize that they'd been acquainted for quite some time.
Hello, Insert Name Here: So, remember how we said this trope was subverted in the Neutral route? It turns out there's a reason for it. The prompt at the start of the game told you to "Name the fallen human". The 'fallen human' is actually the first human who entered the Underground and was adopted by the Dreemurr family. The player character is the eighth human to enter into the Underground; their name is Frisk. The two may share a body, but it's ambiguous as to which one you're controlling at any given time. It's all a bit Mind Screwy.
Love Letter Lunacy: The subplot where you help Undyne confess to Alphys involves bringing such a letter to the latter's lab, but Undyne forgot to actually sign her letter, so Alphys believes the player wrote it and immediately agrees to a date. Hilarity Ensues.
Playable Epilogue: In the Golden Ending, you can go back and talk to the NPCs around the world. Like in EarthBound, they will have unique dialogue to share; most of it isn't too important, but back-tracking to the very first room in the game gives you a bit of a surprise.
Puzzle Boss: All of them, seeing as you have to figure out the proper sequence of commands that will cause them to surrender, flee, leave, lose interest in fighting or otherwise yield.
Self-Sacrifice Scheme/Thanatos Gambit: What the Fallen Child original plan was revealed to be in the True Lab Tapes.
She Cleans Up Nicely: Undyne during the fallen human and Alphys' "date". When not fully-armored, she normally wears simple jeans and a tank top with her hair pulled back, but here puts on a stylish jacket, sweater, and lets her hair down. It becomes obvious that she is trying to look good for Alphys. Also applies to Alphys, who trades out her white lab code for a black and white polka-dot dress during the date.
Wham! Episode: The True Lab sequence. The reveals in this part, in rapid succession: Alphys' experiments with a substance extracted from human souls she calls "Determination", the existence of the Amalgamates, the truth behind the death of the Fallen Child, and how Flowey came to exist in the first place.