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.)