Kyou, Boku no Kanojo ga Saitei no Gesu Yarou ni Kegasaremasu.

Riona Kirishima, who has long been in love with her childhood friend, the protagonist Tōya, finally confesses her feelings and becomes his girlfriend. As a promising star in the track and field club, she is enjoying her school life to the fullest and is at the height of happiness. However, a man appears, attempting to get close to her by exploiting her status as an athlete with a track scholarship, where relationships are forbidden. Around the same time, Tōya’s younger sister, Satomi, is left confused and heartbroken upon discovering that her beloved brother and his childhood friend have become romantically involved. A kind man gently reaches out to comfort her. Life goes on, as these beloved women begin to change—while the one they care for remains unaware

[Translated from Website]