The traditional comic wisdom is that circular character designs imply energy, characters who are very emotional, active, and expressive. (Elmer Fudd, all of the Peanuts kids, Mickey Mouse) Triangles imply powerful, maybe unmatched, unchecked, or dangerous power. (Godzilla, Pyramid Head, that wolf in a zuit suit from the Tex Avery cartoon) A square is literary like the slang use of the word, rigid and constrained. To me, a diamond is a square disguised as a triangle. What traits about this character are you trying to portray?