User blog comment:B214/Volume 23 discussion blog/@comment-172.58.16.1-20170324223136/@comment-31137559-20170325042823

Pawn- Ability to promote to queen, rook, knight, or bishop.

Bishop- Increase in magical abilities, demonic powers, etc.(Typically wizard types)

Knight- Increase in speed and agility.(Typically technique types)

Rook- Increase in strength and defense.(Typically Power Types)

Queen- Balance of bishop, knight, and rook

King(no piece)- Gets set of 15 evil pieces

And saying that Issei's pawn pieces wluld transform into a king piece is ridiculous. That just throws a wrench into the entire evil pieces system. Issei's 8 pawn pieces are tied to Rias' energy or something like that. Those eight pawn pieces are inside Issei but are aligned with her. If those merge into a king piece(which is like turning a hamburger into a hot dog) then those 8 pawn pieces that are tied to Rias would be gone. Those pawn pieces are what makes Issei Rias' servant.