You can actually fight him him after battling Sans but you need to avoid doing the following things:
-Being nice
-Sparing Undyne
-Talking to NPCs
-Playing the clarinet badly
-Waving your flashlight back and forth really fast
-Stomping the ground
-Eating cubed cheese; sliced is safe
-Wearing a sombrero in a goofy fashion (such as upside down)
-Wearing clown shoes
-Wearing a hoop skirt
-Screeching like a chimpanzee
-Running
-Limping
-Crawling
I REALLY don't remember how the exact thing went, but it wqas something like that... 'If something can do something, then fire person can cry' god dang it, I should pay more attention...
A frisky frisk is a scary Frisk.... That horrible, dreadful feeling, of knowing that someone, somewhere, will eventually make a porn version of the game where you either convince the entire underground into having sex with you.... Or you rape them all....
That poor training dummy at the beginning would never see it coming....