Gen will be Jei's host eventually
He was injured by his blade, after all. Originally, the series was going to be much shorter, with Usagi killing Lord Hikiji and possibly dying himself.
Because of the saying that's tossed around a few times early on: A samurai cannot live under the same sky as the killer of his lord or father. Hikiji is both, though Usagi doesn't seem to know the latter (yet.) Senso is the canonical finale.
Sakai plans to keep writing and drawing as long as he can hold a pen. But with Senso as a Grand Finale, if he dies suddenly then the series already has a satisfactory ending. He can continue to have Usagi wander for years before Senso happens, and any continuity errors he introduces in future can just be put down the Unreliable Narrator of Senso. Lord Hikiji is deformed.
He is fully revealed only once in the whole series, and that's in front of Lord Hebi, his most trusted ally.- I believe he's supposed to be the only human character.
