Board Thread:General Discussion/@comment-44061-20130307153642/@comment-5675229-20130411101855

But it is a Zanpakuto! Ichigo has a spirit in his soul that determined that Ichigo is worthy of wielding him, which is a characteristic of Zanpakuto. Zangetsu displays the same things a normal Zanpakuto does; A Shikai, the ability to attain Bankai, and even having an inner world or spirit. While most Zanpakuto spirit info is Non-canon, Renji's has been shown before. Therefore, we know that Zanpakuto having spirits is Canon!

We know that Ichigo's Zangetsu has most of the qualities of a Zanpakuto, and these outnumber the One against it.