Tandeki no Susume

Hina was often unable to play with her friends because of her physical condition, forced to stay home in solitude. One day, Hina runs away from home out of sadness at not being able to join a field trip, and she meets a handsome older boy, Yusei Sakaki. Since then, Yusei often comes to Hina's house to play with her, but as he spends time with the innocent Hina, a dark expression sometimes shows on his face...

(Source: Shogakukan, translated)

Note: Chapter count includes five extras chapters.