Cappy is Kirby who has unfortunately been stuck in Hat form for decades after combining the abilities of two monsters upon his travels. During his torturous existence as a hat, he has lost all sense of self and exists purely to advance Mario's agenda. It is in this madness that he has fostered the ability to utterly enforce his will upon other beings, inanimate objects, and even energy itself, where he then transfers control to Mario.
But hey, I'm just a normal kid, like you, except that I ask questions. And because I'm brave enough to ask questions, I come under scrutinies. Is Wendy using your lunch money to buy heroin? Probably not, but how can we know? I don't want my lunch money going to drugs! Who's taking these drugs? What would be the point? I'm asking questions!