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secret language in a private conversation

Tomoya visits his friend Kyohei's house to study for an exam.
Kyohei's younger sister, Yusa, is also at the house, and the three of them decide to have a study session in the living room.
Tomoya sees Yusa after a long time, and she has grown up to be cute and big-breasted, so Tomoya spies on her many times while he studies.
Tomoya spies on Yusa several times while studying, and Yusa finds out that he is staring at her boobs.
Tomoya thinks this is a bad idea, but for some reason, Yusa seems to be in the mood for it.
She asks, "Do you want to touch my boobs?" Kyohei is sleeping next to her, and the two escalate their activities.