I Don't Cry.

Himeno Aki, youngest member of idol unit "Za-G," is frustrated with the group's decline in popularity, and the other members' lack of enthusiasm. To top it all off, their manager suddenly steps down. Replacing him is Kasahara, a man with a soft smile and an air of unreliability. Former idol and manager; a chance encounter with destiny.

Set in the same universe as union, the two stories are not exactly related, although there is some character overlap.

Source: MangaUpdates