After fighting a few sets of Buggers, you'll find Robo in Proto Dome. Lucca will fix him, and you'll have to choose whether to leave Lucca or Marle behind while you take Robo to the Factory. Nothing there has any weakness or defense against Fire, so the choice comes down to whether you prefer Marle's healing or Lucca's magic offense. Either choice works. You can always return here if you change your mind.