User blog comment:B214/Volume 23 discussion blog/@comment-87.9.174.130-20170319005919/@comment-10330822-20170319012929

Roygun can join whomever she wants. They even said that she's forbidden from operating as a King herself, so it makes sense she wants to join a respectable peerage. Right now, probably one of the most respected devil in the Underworld is Issei. Roygun could join in an attempt to redeem herself, she's starting "from the bottom" as a High-Class Devil.

Also, the whole purpose of promotion is to go from servant to master. But ultimately, all servants still have their masters. Issei's master is always going to be Rias as her Pawn, but he operates as her equal when as a King. Roygun can serve Issei if she wants, just like Ravel chose to do so.

Some devils aren't looking for status and rank, some have different desires. For example, Ravel wants to serve Issei, she doesn't seem to even care about being her own King. After all, nothing's starting her from starting her own peerage right now, she's already a High-Class Devil.

For Roygun is demoted and forced to not be a King anymore, than she has to find new ways to fulfill herself. Not to mention Issei is an up and coming start in the Underworld, meaning that if she joins his team while he is still relevatively at the bottom, she can probably ascend to Ultimate-Class with him.