Dimentio is cunning, charismatic, mysterious, psychotic, and a sadist.

At the start, Dimentio seems like a lighthearted villain, who is a psychopath. Dimentio is very mischievous and eavesdrops on the other minions for information, including Count Bleck, he tends to and often speaks in a condescending or mocking manner and almost never not grins. His speech is pretty colorful, throwing gleeful insults at the heroes and using similes. However Dimentio is very intelligent and good at acting as he successfully tricks everybody and takes pleasure when he does attack his enemies.
Dimentio's voice should appear collected, maybe even calm with a hint of Sadism behind it. A word I think of when I think of a voice for him is "Ringmaster"