Board Thread:General Discussion/@comment-24882930-20160331125812/@comment-27565137-20160401010903

Bambusek wrote: She is most likely the bankai form of manifestation of zanpakto (like Old Man looking different when Ichigo met him with active bankai). But yeah, there is many question about her, how she was out for so long and how she got her own sword (though Isane noted that Yachiru's shikai was unusual)

Exactly! Leave it to Kubo to create more questions than answers.