Board Thread:General Discussion/@comment-11398218-20140521105018/@comment-24854318-20140531042155

Most likely, Yachiru is to Zaraki what Lilynette was to Stark. Stark was so powerful that no one could approach him, so he subconsciously split his power into another being to relieve his loneliness. Zaraki likely did something similar if for different reasons.

We know that he unconsciously began locking away his power after he fought Unohana so that he could draw out and enjoy fights. Yachiru might've been created by Zaraki or even his Zanpakuto to contain that power, which is why she seemingly disappeared when Zaraki awakened his full strength.