Korusuke is a rabbit that appears in the episode A Bandage for a Broken Heart. He is a class pet of the school, and is taken care of by Mami Suzuki. Mami enjoys spending time with the rabbit, partially to remove herself from her disturbing memories of Akito Hayama torturing her earlier in the year.
In the episode, Akito accidentally almost kills it by enducing stress on it. He was trying to make up for what he did to Mami, and so he helps in fetching a vet. When the vet says there was nothing he could do, he calls the vet a fool for not fixing him. Korusuke miraculously lives, and continues to reside on the school grounds.