Starry Sky

Gemini I (1x11)

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It starts off in the past when Kotarou first asks Iku to become a teacher at his school. Once there he meets Tsukiko for the first time and it shocks him that he sees a resemblance between her and his sister Yui. Iku then reminisces on his childhood with Kotarou and how neither of them can get over the loss of Yui.