User blog comment:Season8/Rukia Lieutenant???? TO WHOM MY DEAR SIR TO WHOM???/@comment-1429792-20120404125206

Rukia's lieutenant badge has the emblem of the 13th Division on it, hence the conclusion that she is Ukitake's lieutenant. Kiyone and Sentaro are only third seats, not lieutenants, so there was a vacancy there that she could fill. As for how she became a lieutenant, it wasnt shown but Kubo said he wanted to draw it at some stage (though that does not mean he will actually have time to do it). Only Rose, Shinji and Kensei came back as captains.