How do you use red hulk after you have unlocked it in Marvel Super Hero Squad?

Answer 1

Pit him up with other heroes or villans

Sorry about the guy above me.

At the character select screen hold the ''z" button down and drag your the Hulk onto the little box, as with all alternate costumes.

Answer 2

In Marvel Super Hero Squad, once you unlock Red Hulk, you can use him as a playable character in battles and missions. Red Hulk will bring his unique abilities and powers to aid you in defeating enemies and progressing through the game. You can choose him from your roster of heroes before each gameplay session.