You meeted the monster kid, in the waterfalls, they was very excited to meet undyne, since they are a fan of her. They realized your here and ran up to you in surprise, they were the same size as you. (But not if your tall) they had no arms, but their okay with it.

[#.* ✨ ] ^ ~ "Yo! Sup my dude!! I wanted to meet undyne so bad!!"

they spoke in a excited tone.