Board Thread:Questions and Answers/@comment-9826910-20130618064245/@comment-10630156-20130618233826

The old man wasn't able to overcome all of his powers hence why he became a shinigami and hollow ichigo was able to appear in the inner world as for why he (hollow ichigo) didn't overcome or try to overcome him this is probably as he was simpily waiting for when the old man would step down remember when he said "train him well because one day all that power will be mine" this wasn't meant to be taken as we first thought he was simply saying one day he'd become Ichigo main source of power and that he was simpily waiting while helping to teach Ichigo and saving his life every now and again.

He is probably both hollow and zanpakuto because he is part of one of Aizen's experiments though this was by accident.

Or alterinatively they could have merged because they are made by the same part of Ichigo's mind, his insticts. It would appear that he is a bit further down the path to becoming a true hybrid than the visored.