Board Thread:Fanon Canon - Gotei 13 Applications/@comment-1493741-20141111111609/@comment-1493741-20141112155849

Her history is that she never technically got discharged or left. She was MIA. The way I see it, there was a battle between the Shinigami and Hollows that ended with the Shinigami attacking Hueco Mundo through unstable means. Their forces got there, and Seia was the Captain leading them, the Shinigami were eventually wiped out and scattered and their means of returning home had been taken out, and its instability made rescue attempts both risky and difficult. Essentially she was forgotten and time moved on.

Eventually she manages to escape through (forced) cooperation with a Hollow and decided to retire during the events of the canon. While willing to fight, she also wasn't actively seeking it out, so the numerous arcs where the threats remained in the Human World or Hueco Mundo she stayed put and only got off her butt when the Wandenreich attacked Soul Society itself, keeping whichever Rukongai she was in free of potential Quincy sadists and likely clearing some Soldat on the borders of the Seireitei.

With most of the Captaincy dead, she'd feel it necessary to step in an assist, even if she's only there as a Temp-Captain (Could potentially end up as the head instructor for Shinou Academy...).