Dimentio appears as a Jester which is a type of entertainer/clown with a mask that resembles comedy masks with it's black and white sides. His gloves and shoes are black and are outlined with white. His hat is purple and yellow, it has square-shaped bells at the end, his outfit is the same color as his hat.
Dimentio is a charming and charismatic individual. He speaks very poetically and does not come off as very threatening. However, when his true motives are revealed, he is cold, ruthless and has no regard for life whatsoever.
He will typically make fun of others and mock them in a joking way. He is a manipulative and absolutely psychotic. This is displayed most prominently when Mario and company defeat Super Dimentio. He left behind a slight shadow of his power so he could control the Chaos Heart after his inevitable death.