Given the conversation with McBurn, we know it's something.. similar to his.
If I had to speculate big time, my guess is that Osbourne had something to do with it since he seems to be in direct opposition to Ouroboros. So with all that he's running around and doing, it wouldn't surprise me if he had some access to whatever experimental fuckery they did with McBurn and did a milder version on Rean? We know that, based on Weissman's dialogue to Estelle (when they were trying to tempt her to join Ouroboros), Renne, and what was done to Joshua, they've got some kind of super soldier serum or crap going on in addition to their enforcer training.
I don't think the incident was Osbourne's goons. I seem to remember it being pretty clear it wasn't anything human.
Plus, given that the Schwarzers seem to be in on knowing his background, and Osbourne's line to Rean before leaving him (also the tone). . while part of it could be part of his grand plan, if I had to guess, he had to leave Rean behind out of concern that the kid would be used as a bargaining chip against him. Considering all of the man's schemes and in general how shitty the Nobles could be, I wouldn't put it past them to try to use the guy's kid against him. Plus, we don't know who Rean's mom is. Is she dead? Is he the product of some dalliance with a Noble woman (points if it was Duke Cayenne's wife, ha!).
I'd forgotten about the Walter/Zane fight -- thanks for the reminder. Course, it probably is worth mentioning in SC you *did* have people like Mueller, Olivert, Schera, Joshua (a former Enforcer), Zane, and Kevin (who flat out told Weissman that, at worst, he would have 'struggled a little' when he was power tripping on the Aureole. Class VII had.. Sara.
But it sounds like we're all in agreement about the first two fights. Seriously, I would have liked Duvalie to just be taken down a peg and humiliated, or just have had Rean go full Spirit Awakening and been like 'lolno' to shut that down; even if the gameplay gimmick had been 'can't use Spirit Unification for the next two boss fights'