Nyan Cat

can turn into a rainbow comet.
Ability Rainbow Blast. The ability to extend the mustache and use them as swords.

a grey kitten with a body made from one large pink pop-tart.

Nyan Cat's most powerful enemy was considered to be Tak Nain (also known as "Soul Eater") - Nyan Cat's twin, an evil version of the character.
So Nine is the main antagonist of most of the works with his participation, usually fighting Nyan Cat alone.
Additionally, in Nyan Cat related games, enemies can be dogs, spaceships, or other objects that can harm other Nyans.