He's a human who has turned into a Soul Reaper, hollow, Vizard, Fullbringer, and possibly a Quincy in the near future, his parents are a former Soul Reaper and a Quincy, achieved his bankai in less than a fraction of the time it normally takes and is treated by the Soul Society as some sort of messiah-like figure even though he's far less experienced than any of the captains, and has some of the best plot armor in manga.
Hmmm...Nope, not a Mary Sue yet.