User blog comment:OriginalFanBoy2/Pernida and Mimihagi/@comment-11051098-20150805202207

This question has not yet been answered. I suppose Kubo will explain it before the fight is over.

Theories are that Yhwach might have brainwashed the left arm of the Soul King and it now believes it's a Quincy named Pernida and a Sternritter. Mimihagi was out on its own in Rukongai, who knows what the left arm could have done. No one yet knows how it ended up in Yhwach's service.