Monster Kid is a young monster and major character in Waterfall who stops at nothing to see their idol, Undyne, in action.

Monster Kid is a yellow monster with no arms, a tail, and spikes on the back of their head. Having no arms and frequently running everywhere, they trip and fall often. They have bags under their eyes. One of the bags are bigger than the other. They wear a shirt with two towers and a plane.

Monster Kid looks up to Undyne as a role model. They are an innocent monster who assumes an energetic attitude that pervades most of their actions. They seem to be a bit naive, as well as a bit clumsy, as they trip frequently. They also use words like "Yo," "cool" and "awesome" frequently, usually considered childlike ways of interaction.