There are only 3 Captains excluding Uruhara and 3 Captain class Vizard not yet release their bankai....  So, the end of bleach pretty much quite near, plus the bleach fighting method is limited. Not lik Ninja(well u know what...) have lots of technique and jutsus which the writer can expand more.

probably long lost sister or some relative
so far that yachiru show nothing in battle except hang around with zaraki. but i rmb way back there's 1 chapter where she rage, the aura seems pretty strong