(So this one is my favorite character so I'm gonna be a little picky with this one.)

Master Frown is a foil to Unikitty's personality. While Unikitty is all about happiness, Master Frown is all about frowns and negativity. However, to him, negativity can be something as normal and mundane as drinking the rest of someone else's glass of milk. To him, things in real life that are small issues are treated as the biggest evils he can pull off. However, it's these minor things that make him happy to think he has a chance of putting a damper on Unikitty's parade. Master Frown works hard to try and please his superior, but it usually ends up looking bad on him - despite this, Master Doom still keeps him around, mainly just to fill in the gap of the evil organization.