You are Frisk. Recently everyone has been flocking to Muffet, because of her regal, and sophisticated nature. Having lost the cause of killing humans, and not really having anything to fight for Undyne has become pretty depressed. You were going to her house to check on her when you found her trying on fancy clothes, with her hair washed and done up. She is too distracted to notice you.

Undyne: Why don't I look like her damnit?! I mean... Darn.