Bleach

The Invisible Enemy! Hitsugaya's Merciless Decision (6x21)

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Hitsugaya finds a discovery that helps them in the battle aganist the Arrancar they are battling that it can shapeshift into other souls. And they find out Yui is a clone of the Arrancar. Hitsugaya and group are trying to protect Shōta and figure out a plan to beat the Arrancar.