Undyne  is a fish-like monster that leads the Royal Guard. Clad in her full suit of armor, she pursues the protagonist through the entirety of Waterfall and is frequently evaded or inadvertently thwarted by the Monster Kid.

Undyne has a very outgoing and egotistical voice. She often picks fights just for the fun of it, and is very Gung-Ho. She often has a lot of energy, so don't expect her to ever be calm...