I'm not so sure if Deathfuse really is suppose to be a pirate, I mean, look at him, he looks more like some royal robot.
Of course I think Scervo and Deathfuse have some kind of relation, they are most likely the same kind of robot. Perhaps Deathfuse was the king of some part of the surface a long time ago, and Scervo was a pirate who lived the that same country.
It's very possible that Deathfuse was king of the Ancient Robots, or
is the Pirate King. The fact that he has a hook for a left hand suggests that he either lost a limb in a struggle/war of sorts, or had to challenge another robot in order to gain that title. That's hard to determine since there are only two robots that look fully humanoid in Skyward Sword. But if my speculations are correct in Hyrule-terms, then Scervo is, or rather
was under the command of Deathfuse to begin with.
As for Scervo in SS's current time, I surmise that Ghirahim took control of him and used the Bokoblin buccaneers to crew the Sandship, with him as their captain. If this is the actual case, then Ghirahim had planned and prepared for all sorts of events. Really, all he needed was Zelda in order to complete Demise's resurrection, and Scervo was another means to slow Link down.
Secondly, though, Scervo could have given his allegiance to Demise and Ghirahim, probably for a price. He is a pirate, after all, and I don't think we should solely base that upon his appearance. I can see Ghirahim striking up a deal, which Scervo agreed to. The "trade" could've been the Sandship for all we know.
At any rate, after comparing the two pictures, my best guess arrives at Scervo originally being a subordinate of Deathfuse's. They were probably rogue automatons that kept causing trouble to their cousins, the Ancient Robots, so they were almost wiped out. Scervo and Deathfuse escaped and perhaps are the only two survivors (up until Link encounters them). But, notice the hooks that are on
both of their left arms. Makes me wonder if this was a design of all these humanoid machines, or they both happened to lose their hands. Maybe they were even contenders, and Deathfuse ended up being the victor and subsequently became the ruler.
Regardless, we do know that Scervo took command of the Sandship and became it's commander.
And Deathfuse somehow wound up in Sky Keep. Hylia most likely placed him there to test the Hero, as well as protect the Triforce piece that he guarded. Again I'm puzzled by the lack of knowledge about him. Was he loyal to the Goddess of Time, or was he sealed within the Keep against his will?
We can only use our imagination to piece their relation together, until Nintendo releases an official statement, if ever... Because I'm not sure anyone can know the lowdown at this point.