Special mention goes to Cartman, who really opposed to the idea during the Black Friday trilogy, (which is the prequel to the game) not because he was playing a girl, but because he didn't want him have a noble title. Now, he finally recognizes him as a princess, and allows him to rule by his side.
Karen, Kenny's sister, calls Princess Kenny "her sister" in dialogue she gives to the player character. Awww. (She also tells you that Kenny thinks you're cute.)
Kenny keeps fishing for likes on Facebook, posting "Who's the fairest of them all?" wall posts. He gets tons of likes instantly.
Cartman's rewarding the New Kid with new titles as thanks for his help. Seems like Cartman genuinely likes the New Kid, huh? If you decide to side with the elves during the School Assault, Kyle will say he regrets ever doubting you and awards you with your promotion to Commander. Cartman still promotes you to King.
If you end up fighting Stan and take out his dog Sparky first, he'll cry "No, no, NO!"... and get an Attack Up.
It helps that the buff is called "Stan Is Mad As All Hell". He's legitimately upset about.
A blink and you'll miss it moment; when fighting against Nazi Zombie Princess Kenny one of the special attacks involves summoning the Unicorn as a Nazi zombie. Stan decapitate's its head if the attack is performed successfully, and afterwards looks guilt-ridden for killing an animal. Trey and Matt did not forget he's a Friend to All Living Things.
Also if you end up fighting Stan, if you don't take out his dog Sparky first, when Stan is almost dead, Sparky will lick Stan's face and play with him and Stan's health restores.
Nazi Zombie Chef coming to his senses. His friendship with the kids was so strong that it overpowered the Nazi Zombie virus. Sadly, Clyde lights him on fire, forcing you to use a fart to blow him up.
After years of taking crap and being beaten up by his sister, both the player and Stan can finally get some payback and beat the ever loving shit out of Shelly. And yes, it is as sweet as it sounds.
Bonus points if you don't just immediately slay Shelly with a summon (she's not considered a boss, even though she's incredibly tanky and has REALLY good attacks) and get the last strike as Stan.
It's pretty sweet that even though Clyde is making an evil army of the dead, when the boys confront him they act concern for him as well. And even though Clyde and his army of darkness are the "evil" one's of the game, Kyle still refers to them as "our friends."
The boys throwing the Stick of Truth into the lake, banishing the artifact into the void for all of eternity. Pretty sweet when they consider their friendship more important than something they've been viciously fighting over for so long.
During the assault on the tower, Ike the Pirate Captain will launch a firework. He's stopped by a Cyclops and punted to the ground. Once you launch the rocket and take out the cyclops you get a friend request from Ike.
Playing Hide and Seek with the Kindergarteners.
Ike can be found playing pirates in the playground behind the street the boys live on. If Stan or Kyle is with the New Kid as he walks through the park, they'll both greet Ike with "Ahoy, Maple Beard!"
Hearing Ike say "I am the Pirate King!" is its own kind of adorable.
At the final battle, Kyle will use Ike in order to stop Nazi Zombie Princess Kenny from healing himself. After the attack is done Ike runs off crying and Kyle immediately runs after him to comfort him.
When the group runs into Randy during the final dungeon, he happily calls you "My favorite kid!". The fact that he does it right in front of his son makes it a little less heartwarming, though.
After the New Kid tells the gang off and leaves, for any given reason, it's nice and sweet to see him playing with Stan, Kyle, Kenny, Cartman, Jimmy, and Butters again the following day, suggesting that they made up and are still friends.
The player character's mom is a really nice woman, and one of the sweetest moments is when she tucks your character in after their first day in town.