Technically, only Ken is confirmed alive, but anyone whose body wasn't shown is likely to return. Skipper and Tiger were given fatal enough offscreen accidents (jumping from a train in the middle of nowhere with two bullets in your chest, firing a rocket capable of blowing up a train directly into your crotch) that they probably won't show up again. With Angie, we know she was in a bad way, but we might expect that she actually had more agility than she let on (based on the ball trick). As for Twenty Faces, of course, we never see anything actually happen to him. And a man like that must have big plans. I suspect he'll return, years later, once Chiko has become a detective. And then student and teacher will face off as enemies.