Monster Kid

A devoted fan of Undyne, Monster Kid is a young monster in the Underground that will stop at nothing to see their idol in action.

Appearance
The kid is a yellow monster, with a tail and spines on the back of their head. They have no arms and wear a striped shirt. The marking under one of their eyes is bigger than the other marking. This is likely a black eye from falling on their face often.

Personality
The kid looks up to Undyne as a role model. They are an innocent monster who assumes the best of the protagonist. They're characterized by a spunky, energetic attitude that pervades most of their actions. They seem to be a bit naive. The kid seems to be a bit clumsy as well, as they trip frequently.

Despite being notably frightened by the protagonist during the Genocide Route (once they discover the protagonist's intentions, at least), they do an intense expression, ready to protect the Underground.

Main Story
The kid meets the protagonist in Snowdin, pointing out how their striped shirt shows that they're a child.

The kid later sneaks out of Snowdin to explore the Waterfall, looking for Undyne. They follow the protagonist, not knowing that Undyne is searching for them as they are a human, and feel a little jealous that they receive so much attention.

Neutral Route
The kid eventually finds out that the protagonist is a human, and reluctantly considers them an enemy. They try to say something mean to the human, attempting to create a reason for the two to be enemies, but feel bad for it and begin to leave.

Just as the kid takes off, they fall and hang off a ledge. Undyne approaches and the player is given a choice:
 * Help the kid up the ledge
 * The kid defends the protagonist from Undyne, who backs off. The kid decides that they'll remain friends and leaves for home.
 * Let the kid fall
 * Undyne dives down after the kid to save them, she is badly hurt but refuses to rest. The kid tells Undyne that she was right about the human, who just stood there waiting for them to fall.
 * Run to Undyne
 * Undyne saves the kid and is badly hurt. The kid tells Undyne that she was right about the human, who was too eager to fight her to bother saving the kid.
 * Run away from Undyne.
 * Undyne helps the kid up, who tells her that the protagonist was just going to get help from others.

Pacifist Route
After fighting Undyne, Monster Kid can be found in Snowdin. When talked to, the kid says that now they think Undyne is not that great and because of that they found themselves a new idol (who, quite obviously, is Papyrus). If saved and befriended, the child will grow to idolize Frisk also. They are seen cheering the human onto victory as they face off against Flowey. Monster Kid is also seen during the credits, attending Toriel's school.

Genocide Route
The kid is warned by Undyne that the protagonist has hurt a lot of people. They approach them anyway, not believing that they would do such a thing. The protagonist silently approaches the kid with a 'weird expression'. The kid nervously stands their ground, and the protagonist eagerly enters a battle with them, declaring that they are 'in [their] way.'

The player cannot flee from the kid, but they can attack, check or spare them: If the player chooses to attack, the kid disappears for the rest of the game. It is assumed they evacuated with the others.
 * If the player attacks, Undyne takes the damage instead, depleting her health. The kid escapes, whilst harboring a shocked expression.
 * Checking the kid suggests that they look like 'free EXP'.
 * If they spare the kid, they will say that they knew the protagonist wasn't that bad before they leave. This ends the genocide run and the game continues on the Neutral Route.

Trivia

 * The kid has a sister, as mentioned if you agree to say something mean to them in the Neutral or Pacifist Route.
 * In Waterfall, if you talk to the Echo Flowers, you can follow the conversation of someone and their sister. This may be Monster Kid and their sister.
 * Toby Fox has stated that the Monster Kid's name is Kid.
 * Monster Kid's design may have been inspired by the Pokemon, Zuruggu (Scraggy in American localizations) . Both are small, yellow, bipedal lizard monsters with constantly protruding teeth. While Zuruggu (Scraggy) may have arms, they have to perpetually use them to hold up their pelt, which is a lot like not having arms at all, just like Monster Kid does not have arms at all.
 * If Monster Kid (MK for short) did have arms, they would use them to flick off the person who tries to edit that previous entry.
 * Monster Kid was designed by Magnolia Pearl, who also designed Jerry. They run a webcomic titled Monster Pulse.
 * Monster Kid was inspired by the show Henry Hugglemonster.