Some series had created over the years some spin-offs series http://en.wikipedia.org/wiki/Spin-off for example the Angel Links OAV is a kind of spin-off of Outlaw Star if there was a Spin-off series of OP which characters may be the main star?
On my side, I taught of:
Hina with Fullbody and Jango
Johnny and Yukasu
Ninjim with Pipman and Tamanegi