Kashi White

"White" is part of Zangetsu. Specifically, the portion of Kashi's spiritual energy that forms his Hollow and Shinigami powers. He was there the whole time, and he was becoming strong enough to temporarily break the "Old Man's" control.

Kashi went to the Vizard for help, based on the logical assumption that a shinigami with Hollow powers could help him control his own Hollow manifestation. But Kashi is a unique case.