This is an idea, or basically an AU, that I've had in my head for a few months now. I'll call it: double mind au.
It's about Sanji having multiple personality disorder (or something similar enough to be considered as such), and this other personality, let's call him Stealth Black, Black or just Three, is what Sanji was supposed to be if Judge's experiments had succeeded.
The easiest way to tell that he's in control of the body is that Sanji's hair turns black and the irises in his eyes become a paler blue. Furthermore, both Sanji and Stealth can be aware of what's happening despite not having control of the body at that moment. Overall, Stealth isn't evil, more of an extremist than anything else.
His primary concern is keeping Sanji safe, though he's very obedient to those he considers his superiors. He has minimal emotions and doesn't possess the genetic enhancements he needs until Sanji acquires them in Wano. At most, Stealth only experiences a considerable increase in strength when he's in control of his body. He thinks very logically and doesn't tend to react much, which makes it difficult for him to sense other people's moods or relate to them. He first appeared after Sanji's brothers found him in the dungeons and beat him. During the attack, Stealth manifested and tried to defend himself. He failed, but since he doesn't have many emotions, he didn't seem to care much. No one noticed that his hair turned black due to his helmet and the lack of light in the dungeons. This happens a few more times without anyone noticing.
When Sanji escapes from Germa 66, everything goes well until the day they reach the rock. Sanji starts feeling extremely hungry, and Tres takes control for a time, alternating between them as the days pass. Stealth takes the knife and threatens Zeff, but when he realizes the sacrifice Sanji made, he immediately takes control and starts berating the chef. Zeff doesn't understand at first because of the physical change, but he doesn't seem to care at the time. After being brazen, Stealth doesn't take control again until Sanji has a nightmare. In the most logical response, Stealth takes him to Zeff and snuggles up to him without asking any questions before returning control to Sanji.
Now Zeff is truly confused (the first time it happened, he thought Sanji was just hallucinating), but he still doesn't care much as he hugs Sanji, who is embarrassed and apologizing for both himself and Stealth. Zeff just silences him, saying it doesn't matter, that he should just go to sleep, and that he needs more than this strange feeling to make him leave.
