Kicking butt with an 8-bit character (Simon or Fuuma).
Charlotte when someone else gets charmed by Astarte.
"You're dead!"
And its rarer variation...
"You're ALL dead!"
If someone you were playing with is using a DLC character you haven't unlocked/bought yet, they will show up on your screen as an Axe Armor with no remote attempt made to give the sprites any animation besides "walk" and "throw axe". The limited animation fudged to convey complicated things can be gut-bustingly hilarious for the person watching.
Death's English laugh in his Chapter is hilarious.