Antonio is the first boss you battle in Streets of Rage, most likely the easiest as well. You fight Antonio in the mean streets of Round 1, where after fighting a few enemies, face Antonio himself.
Antonio only has two attacks: A kick from close range or if you're further away, he'll use his boomerang. His boomerang can be punched away if you get the timing right as well, but he will spawn another one. He also appears in Round 8, wearing green attire.
Antonio himself, however, doesn't make any second appearance in the sequels. In fact, the only second game he appears in is Bare Knuckle Mobile.