User blog comment:Crimson and Darkness/DxD Updates 2~/@comment-27083298-20180120112615/@comment-29963364-20180120140543

In the LN, Issei made it very clear how the Triainna forms looked like. It's really just his regular BB Armor, with some added  features based on which form it is.

Bishop: Regular Armor + Backpack with two cannons mounted on it.

Knight: Regular Armor, just thinner, with some parts shedded off.

Rook: Regular Armor, just thicker, and with elbow-pistons.