Houkago, Koishita

Briefly about Houkago, Koishita manga:

Koishita Kojiro (Hayama Kao) works part-time as a temporary manager of the boys' volleyball club. Her older brother, an alumnus who's currently a teacher at the school, advised her to take care of student council member Kuze Nagisa. Kao tried her best but it doesn't seem like Nagisa is interested in being in any club... until one day, when Kao found out about the secret training behind the scenes!