One - Ai ni Naritai

Sachiyo Yoshimoto is a first-year high school student. Due to family circumstances, she lives at her cousin Keiko's house, but she is active in the swimming club. At first, she longed for Kota-senpai, but when she noticed the kindness of her classmate Takumi, a faint love blossomed. However, Keiko also likes Takumi, and Sachiyo's love is in trouble!

source: mangaupdates.com/series/u0iwlrd/one-ai-ni-narita