Oneesan

Oneesan is the heroine of the story. She is the calm sister Boku-kun meets at a bus stop in the countryside on a rainy day. She is very tolerant of ecchi and rather actively initiates and leads Boku-kun in doing ecchi with her. She also lactates. The CV will be played by Natsuki Nogami (野上 菜月) in the first part and Mei Goenou (御苑生 メイ) in the second episode of the middle part. (Source: ch.dlsite.com/matome, translated)