It

A being simply referred to as It, it has no name or confirmed gender, or even species.

Biography
It was created by Mayuri in an attempt to better understand spirits, more specifically, the different powers Hallows exhibit. During the process, Mayuri pushed one too many boundaries and the result was It. Without hesitation, it smashed its way out of the Soul Society and began it's rampage.

Powers and Abilities
Though It holds a zanpakuto, the weapon is merely decorative, not a true soul cutter. As far as powers go, It has intense physical speed and strength. In terms of powers, It has the ability to mimic the skills of anything it kills, in addition to absorbing the spirit energy of It's victims. When powerful enough, It can send out a ethereal shadow-like entity to fight instead.