User blog comment:B214/Shin DxD 1 discussion/@comment-2601:340:C501:C683:D9E9:7B80:4551:9FCA-20181113211020/@comment-31137559-20181113220141

Issei will always be Rias' pawn unless she trades him to someone else. As a reincarnated devil, he is always going to have those evil pieces within him.

Issei is still Rias' pawn but he's not her servant as he's his own king now. He is still obligated to participate in Rias' Rating Games as he is her pawn, but other than that, he's independent.

Devils are registered as kings. A king piece is not used. If it was, then it would conflict with the evil piece(s) reincarnated devils like Issei have already in them. The king piece is something entirely different.

Issei and Vali have never made a decision about becoming Maou so we don't even know if they'll accept the position if offered. And there's also no guarantee of them making it to the Final 4.