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In Hanako of the Opera, Teru says Kou asked him to handle preparations for the villa. He mentions there's a beautiful lake nearby it too. The villa is seen in the end art with Mitsuba and Kou with them walking in the lake.
TDLR : Mitsuba and Kou ran away from the opera together.
Kou is almost always seen or implied to be with/talking to Mitsuba in AidaIro’s AUs.