Kazuto is Yukine's dead brother.
Kazuto was a delinquent, but was sweet to his sister. He died in an accident escorting his friends home. Yukine and her delinquent friends refer that Tomoya's personality is the same as her brother. His death is the reason how Yukine gets close with the delinquents, who were Kazuto's friends.
He did not make an appearance in the anime, though Yukine mentions her brother and her looked alike.
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