Cosindering Kitase and Toriyama said there's going to be a lot of new characters, I'm leaning over towards the Pulseworker as being a Guest party member. You probably activate it in a dungeon(like Robo in Chrono Trigger) and he obeys you. They are probably going to do the sappy "robot sacrifice" bit where a powerful monster's going to try to kill Serah and the Pulseworker will jump in front of her, shielding her form the attack, but taking major damage in return. After that, Serah will say a prayer at grave marker she builds out of his parts.
But yeah, in all seriousness, I do think it's a party member, much like the Worker 8 robot in Final Fantasy Tactics. Besides, I'm all for non-human party members! Have we even had a robot party member in a FF game? That would be awesome!!