Diablo Wiki
"Michio Okamura was our character artist, and just loved working with him. We went back and forth with character designs all the time. He designed the original Diablo himself, and most of the monsters. We actually hired him when we were doing superhero games, and I don't think he had any job experience before that. We were doing the Justice League Task Force game, and he could kind of draw superheroes. So we kind of based that art on his drawings, and then he transitioned right into the Diablo stuff really well."

Michio Okamura was a character artist at Blizzard North.

Okamura was hired by Condor when they were working on Justice League Task Force. He continued to work on the Diablo series. Among his character designs was the original Diablo design.[1] He did the majority of character and monster concepts.[2]