Board Thread:General Discussion/@comment-4317207-20130313091731/@comment-7159755-20130314081856

I think it's reiatsu remained inside isshin in some form or another, which eventually moved onto ichigo, which would explain why it's blades reseble Zangetsu's in a few of it's forms, and why it's shape resembles a nice mix of ichigo's hollow forms, collar, horns, mask, lines, spikes, reverse jointed legs, etc. and which would lead to his fullbring abilities as well.