The Subspace King is as his name suggests, the king of Subspace and the father of Tabbuu and Drabuu. He is the centre of all evil, he says he is the voice that whispers into peoples ears to do bad stuff. As he's king he's the highest authority in the Subspace World. He also creates 'natural' disasters when he feels like it. He's that powerful.
Subspace King seems to loathe everyone and everything in creation, exept his sons, his army, the Hands and the Ancient Minister. Well, I say sons, but actually, he seems to only pamper Tabuu, since he takes after himself, and takes all his anger out on Drabuu, since Drabuu sometimes helps the heroes by stopping some of his more dastardly plans. But all in all, Subspace King is basically emotionless apart from hate and loathing.