Kubo confirmed in a Q&A on his website that "Yachiru Kusajishi" has been a manifestation of Zaraki's locked bankai all along. It's unknown why she could interact with others. She had shinigami mimicking powers only because Zaraki imagined her as one. She has disappeared now because Zaraki finally unlocking his bankai resolved the situation with her. Evidently Nozarashi (her real name) manifested as 'Yachiru' in an amnesiac way, sheerly out of desperation to have a communicative relationship with her wielder.
