A character isn't necessarily a Mary Sue simply because they're powerful. Otherwise, we'd literally only be able to write stories about generic, boring everyman-types.
A character is, in my experience, only a Mary Sue under two conditions:
1. Their actions never have any consequences, or if they do, they're greatly reduced compared to those of literally any other character.
2. The narrative bends over backwards to ensure they're always right and victorious.
Considering what I've seen from
@JohnSolaris up to this point, I highly doubt he would be irresponsible enough to let his character's abilities and strength trample over his desire to run a fair game.