Play As Old Red:
Beat career mode with Red Rocker to unlock the original red robot.

Play As Old Blue:
Beat career mode with Blue Bomber to unlock the original blue robot.

Play As Ben Hurt:
Beat career mode with Slamuri to unlock BEN HURT.

Play As Sir Haxalot:
Beat career mode with Head Case to unlock SIR HAXALOT.

Play As Tywrenchula:
Beat career mode with Stakeout to unlock TYWRENCHULA.

Thanks to Revolution reader Robot Exterminator!