Board Thread:General Discussion/@comment-24815992-20140619090507/@comment-29325824-20190720145819

I'm not sure, if they did provide an explanation for it I probably missed it somewhere or it got lost in the fan translations (hence why I agree with you guys not to rely too blindly on them). I have to admit I haven't read all of them yet, as it's not easy finding decent ones that aren't just summaries. But CFYOW does imply that there is more to Zanpakuto than we think, as at one point Kazeshini reveals info about Zangetsu and Nozarashi that Hisagi himself did not even know (trust me, I don't quite get it either).

That being said, I think I can answer one of your questions: how she was visible. Wasn't it stated in the manga itself that materializing your Zanpakuto spririt is not only possible, but necessary to achieve bankai? Well, due to Kenpachi's potential, he has been subconsciously materializing her the entire time, which also explains why he achieved bankai so suddenly. He just needed to be aware of it and stop holding back.