When you speak of Voltron, I think the animated series on Netflix.
Voltron is somewhat tricky. On one hand, they're gear that characters pilot, but on the other, they are also somewhat intelligent and can act independently. I don't know how intelligent; they never talk.
I don't think Pathfinder has any rules for building or piloting giant robots. I know that D20 future had some. You could design robots and even make them capable of combining. There was a Mecha Crusade chapter in Dungeon magazine 95 with Polyhedron 154 (hybrid magazine). It has character classes and even had build points by level.
I'm not sure of what level to start the characters at. Voltron and their individual robot lions are much more powerful than 1st level gear.
The thought I had was as pilots outside the lions, they are individually normal characters. So campaigning can be regular campaigning stuff. Hell some mid level quests can be hunting down the keys (I know a voltron doesn’t use keys but this version would) more about keys below.
For each lion I’m thinking skinning clockwork dragons as lions, give them both the advanced and shield guardian template so each Lion would be a CR 18
My thought process is the key binds a pilot to the lion. So no key no forming Voltron. As far as intelligence goes Id have it where a pilot could summon from a distance and the lion would react. The Keys would be able to communicate via empathy and are lawful good. Won’t react to anyone on the neutral or evil plane holding them. Won’t actively attempt an ego save but won’t respond, won’t activate lion, etc. another control would be giving the key blind sense so it won’t allow the pilot to call the lion unless an adequate threat is presence.
I'd track down a copy of "d20 Mecha" (originally for BESM d20, IIRC) and use that. not perfect but probably the least hassle for doing giant piloted robots in th d20 system. I'm pretty sure it doesn't have rules for combining mechs but it has pretty much everything else, and you should be able to hack something for a combined form fairly easily.
I do not know the character, but from a quick Google Images research I gather it's a robot.
My suggestion is to simply use an Adamantine Golem (CR 19). Add the Brain Bioconstruct Modification to give it feats (CR +1), and slap a bunch of HD and stats increases to make its CR appropriate.
Controlling the golem can be done via the Control Construct spell. Since you were thinking about adding a key, then just add a magic item that can be used to cast Control Construct at will, but attuned exclusively to this golem. The "pilots" will then need to keep concentrating to operate the golem, and unable to do anything else. Otherwise, just come up with some two-pieces amulet that needs to be worn to impart commands on the golem.