User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170410191626/@comment-10330822-20170411204755

Exactly, and her being a High-Class Devil doesn't matter either, because Ravel was a High-Class Devil too, and she joined as a Bishop. Ravel didn't need to do that, she just could have started her own peerage and served Issei. Just because you are already a King doesn't mean you can't join a peerage and get a piece. In fact, if Issei gave his Rook piece to Roygun, it would probably make her even stronger.

So it wouldn't be a waste of a piece. What would be a waste is making some random weak female a Rook and then being forced to train them up to High or Ultimate-Class like all the girls are already at. Issei needs powerful females "now" he can't take the time to train low-class devils.

I wouldn't discount Roygun joining, she's powerful and female, which are the two real requirements for Issei's peerage right now. Issei's peerage is incomplete but he doesn't have the time to take some girl from Low-Class up like Rias did for him, hence why Elmenhilde isn't much help right now. He's already fighting Satan and God-level enemies, which means his peerage choices need to already be powerful females.

Besides, its also assumed that all of these girls are going to be High-Class and have their own peerages in the future anyway. So Roygun already having one is kind of irrelevant, it just means Issei has more allies in the short-term.

Also, she doesn't have to join permamently either, she could just join for the cup matches.