Kidyne

Kidyne is the friendship between Monster Kid and Undyne from the Undertale fandom.

Canon
Since Monster Kid meets the protagonist in Snowdin, it can be assumed that it lives there. Sometime before meeting Frisk, Monster Kid had heard of Undyne's feats and begun admiring her for it, to point where it wishes that it could be just like her someday.

Neutral Route
In the Neutral Route, Undyne is hunting the protagonist purely to harness their soul for her superior, King Asgore, the Monster Kid is completely unaware of that and tries to befriend them purely because they're a kid like them. On their way to Waterfall, Monster Kid follows Frisk in hopes of meeting Undyne there, having heard that Undyne is hunting a human unaware that Frisk is said human, believing them to be a fellow monster. Upon seeing Undyne attack Frisk, the two of them hide from her in the tall grass. When Undyne thinks that she captured the human, she immediately lets Monster Kid go when grabbing it by mistake and looking for the human elsewhere. Once Undyne leaves, Monster Kid admits that its a little jealous of Frisk for receiving Undyne's attention.

But when Monster Kid finds out that Frisk is a human, it will try to (verbally) attack them, only to trip when they see Undyne approaching and nearly fall from a ledge. Frisk can then make a choice of saving the kid, let Undyne save the kid or run away from Undyne. Should Frisk save Monster Kid, it will back them up against Undyne. Should Frisk let the kid fall, Undyne will save it, Monster Kid will continue to respect Undyne but hate Frisk from now on for not bothering to save it. However, should Frisk run away from Undyne, Monster Kid will tell her that it is sure that Frisk was just going to get help when Undyne helps it back up.

Pacifist Route
Regardless of what choices the player had made during the Neutral Route, Monster Kid will join Undyne and the others cheering them on in the final fight. After the fight, Frisk can go back to Snowdin to hear Monster Kid admit that they stopped idolizing Undyne if Frisk was the one to save them.

Genocide Route
Due to Frisk becoming a notorious murderer, Undyne went out of her way to warn Monster Kid that they cannot be reasoned with and should be fled on-sight. Naively, Monster Kid will approach the human, only to get stared at with murderous intent, leading to Monster Kid trying to act like Undyne and put itself into Frisk's way. However, upon trying to attack, Undyne will rush in and take the blow for the kid, slicing her in half, but given enough time to warn the kid to run away. However, instead of dying on the spot like everyone else, Undyne will resurrect to give the human hell for trying to hurt an innocent child. Since Monster Kid can never be fought again, Undyne at least succeeded in protecting it, though just like everything, the ending renders this somewhat moot.

Fanon
Given the canonical admiration Monster Kid has for Undyne (unless the player intervenes), some reflect this in fanwork. However, Monster Kid is not a very popular character in fanfic, so people tend to focus on Undyne's other relationships in the game.

Fandom

 * FAN FICTION

Trivia

 * They both appear in Deltarune as Hometown citizens, but they don't seem to interact as much.