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Due to some random Arrancar chick, Ichigo and Grimmjow are cursed. During the day, Grimmjow is a cat. For one hour at dusk, he and Ichigo are both cats. Then, for the night, Ichigo is a cat. But at dawn, for one hour, they are both in their normal forms. And, according to Urahara, the only solution is for them to fall in love.
So, nothing to worry about.