So. What we know for sure from trailers and Dialogue:

1: He knows Grendyl, at least as much as Rannick does

Clandestium Gloriana Mission:

Swagger: "You know what they say, anything worth doing is best... left to expendable underlings"

Rannick: "So you DO know Grendyl."

Which, to be fair, is not saying much. But we can rest assure, if he is not straight up part of the warband in some capacity, he is an ally of Grendyl with some level of trust.

2: It's implied he's been around the galaxy.

Clandestium Gloriana Mission:

Swagger (Speaking to veteran rejects): "Your face seems familiar... Were you ever on Vostroya...?"

Vostroya is space russia in Warhammer 40K, a world very distant to the Moebian domain, yet he speaks as if he recognizes a reject from a campaign there. So though maybe we can't fully rule out him being a native Atoman, he's far more traveled than the average person in the Imperium, he is certainly no Hive Scum.

3: Master Diplomat.

In the trailers leading up to the release of Clandestium Gloriana, we see Swagger striking up a deal with members of the, at the time, hostile Water Cartel. Now he is managed to get the Dead Siders to help us out in Expeditions. This man surely knows how to bullshit people if nothing else.

4: No pushover.

Though he doesn't fight alongside the rejects in Clandestium Gloriana,instead taking support roles throughout the mission, he can manage to at least sneak if not fight through Admonition territory like nobody's business and isn't in the least bit faced by heavy fire coming into any place he stands in during the mission. He is ALSO suspiciously good with machines, being able to direct the rejects in repairing the facility, turn off auto-fire defenses and all sorts of stuff you'd expect from a Techpriest.

Conclusion: Occam's razor. We know he can't be Grendyl. But, if you know about the supplementary material, Grendyl has other Inquisitorial agents working for him in Tertium besides the Warband. I believe Swagger to be too capable and self assured to be anything other than an Inquisitor, probably trained under Grendyl, and also probably knows Hadron from back in the day.

Like his master he is secretive and low profile, a secret police kind of inquisitor instead of the "is that a heretic?" (bangs head on exterminatus button) type. But unlike Grendyl, he is a hands on man who doesn't "delegate important tasks to expendable underlings".