Board Thread:Questions and Answers/@comment-11927664-20130730160308/@comment-26269700-20150402101236

It is possible, because the series has not gone into enough detail about reincarnation, except for the one filler of the dog, my theory is that when Rukia stabbed her shikai into Kaien, his soul split into 2 places, in Hueco Mundo, and within Rukias shikai, this could explain the time when Kaien told Rukia that part of him will be with her, maybe he wasn't just being literal. Then when Rukia stabbed her zanpackto into Ichigo, the merge of her soul with his, created his Zangetsu. Although this is just a theory an Anime Theory.