I Don't Wanna Cry by cherithangel

There were four of them. Yunho barely wanted one, and now he finds out that there’s four teenage boys standing on his doorstep. 

Yunho finds himself adopting four boys after his father's death. It quickly becomes apparent that they had been abused and he works to find a way to help the boys return to a normal life. A really good story.

Another good one by her is:

The Way U Are M

DBSK is a rock band, but they need a lead singer. Enter Jaejoong, a completely innocent boy who makes it into their world, only to realize that all the members are -as-hell- and they've got their eyes on him!

Exactly as it says, between poor Jae and everyone, but then Yunho makes his move and...