... No, you know what? SCREW IT! WHY SHOULD I TELL THAT STORY WHEN YOU'RE ABOUT TO DIE!?! NGAAAAAHHHHHHHHHHHHHHHHH!
— Undyne before being fought
Undyne (/ˈʌn.daɪn/, UN-dyne) is a fish-like monster that leads the Royal Guard. Clad in her full armor, she pursues the protagonist through the entirety of Waterfall, who is also frequently evaded or inadvertently thwarted by the Monster Kid.