Let's Voice UnderTale! (playthrough with voice acting!)
taikunoaku for Monster Kid
This kid is Undyne's biggest fan. When they're not gushing over how cool Undyne is, they're stalking Undyne. This voice can be fairly freeform.
*bursts out of the bushes as Undyne is about to ambush the player*
Undyne!!! I'll help you fight!!!
*does a double take at Undyne and the player*
Yo! You did it! Undyne is RIGHT in front of you! You've got front row seats to her fight!
*does another double take*
...
Wait. Who's she fighting???
*Undyne grabs Monster Kid by the cheek and drags him off screen*
Hey! You're not gonna tell my parents about this, are you?
meatwad?
Ssssshhh, don't tell. I'm not that good at kid voices.
Not really fitting for the Monster Kid, but It's really entertaining. There has to be a monster that would match that voice! Has to be!