@maxterdexter:
Drake being a mole makes Law being one very unlikely, getting so close to Doffy and not sealing the deal.
Not to mention that we know Law wasn't scouted by Marines. He formed a gang in Swallow Island with Bepo, Shachi and Penguin.
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@MiyamotoMusashi:
akainu is painfully lacking a backstory.
But i doubt a military branch would have that as requisite. It can certainly make your role more credible but it won´t be a prerequisite.
It's a requisite. It was established in the very first chapters that people with ties to piracy are forbidden from enlisting. They make background checks.
Koby only managed to get in because Ripper was grateful enough to Luffy to turn a blind eye.
Edit: Wait… I think mistook your point.
But still, I think it makes sense to have people with piracy experience in spy roles. They would know first-hand how pirates organize or coordinate themselves, picking up dialects, practices and stuff.
You're downplaying the "credibility" part since that's essential to espionage work.
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@Sengokusgoat:
X-Drake's box starts with 'Navy HQ', so to me it seems most likely they ultimately answer to the guy in charge of Navy HQ, i.e the Fleet Admiral, although perhaps not directly.
Navy HQ tends to be used for Marines that actually work at the HQ. That's about it.
They are said to be better-trained and stronger soldiers than branch Marines.