If I get this right, the character concept is "guy who can control battle doll"?
I would let the character buy the powers you want the machine to have and attach it to "human form" plus "item of power". And he would need a power from the "beast change" family, so that he can remote control the doll. This would also let him change his skill setting to the skill setting of the doll.
The thing that would be bothering me is, that the character would be entirely shielded from any damage, but I guess that would be ok, because he would still be taken out of the fight for now. And he would have to repair the doll to be able to get back into a fight after that. The doll should probably have a +0 power "it's a machine" (similar to living dead), which would mean, that it can't recover stress or consequences by resting, it has to be repaired using craftsmanship.
So you have the character himself, who would only have "control doll" as a power, the rest would be linked to the item of power, namely the battle doll. The doll would get it's own sheet, but it would mechanically be the "beast form" the character can assume. With the beast form come additional powers (inhuman strength, speed, etc.) and it would have its own skill tree (like any beast form), so you would not use discipline to control it, you would use the the skills as usual.