My Ex Is My Stepbrother - Let’s Continue What We Stopped Back Then

The chapter opens with a flashback to a time when the protagonist was a member of a track and field team, and he saw the manager of that team, a senpai named hayama, with a young woman, whom he thought to be "childish," but who turned out to be a "more mature person" . The protagonist feels that he has seen the girl before, when he saw her with the senpai, but now he realizes that the girl is not the same girl, but a "younger person," and he is happy that he finally knows how senpai feels when he is with "a more mature person," like hayama