Once obtained, Steve can be accessed in Dungeons by pressing R3 when controlling Max. You'll then switch to the Ridepod. Fighting with the Ridepod isn't very different than fighting with Max and Monica. Left stick to move Steve around, X to attack with your current Arm, R1 to block against attacks. You may think Steve is better than Max at Chapter 1, which is true, but in order to keep Steve strong you'll have to invent new parts for him, which can be time-consuming, especially if you do not use your Camera very often.