Expanding on this because I hit submit too soon.
The geass that Lelouch and Rai have allows them to demand control over a particular person, only once. If you look at Lelouch, he is the vital piece. He plans, he schemes, he inspires his forces to victory. However, he, himself, is slow and weak. He is not an effective fighter.
The Emperor seems to be the most powerful. His geass offers a great deal of control and power. His movements are the most frequent, representing the Queen, which has the true power on a chess board.
Rollo moves in odd ways--seeming to teleport here and there. The Knight is a unique piece in terms of its movements, allowing it to bypass defense and offense alike.
The Pawn is the weakest of all pieces, used by the more powerful figures. However, when all obstacles are defeated before the Pawn, and it manages to grow, undeterred, it has the power to become anything it desires.
The Rook and the Bishop both fit Mao's sort of radical power. In both cases, releasing the Rook or Bishop requires a bit of movement on the part of the pawns. In this sense, you can accuse the Rook and Bishop of being pieces that do not reveal themselves until the middle of the game. Depending on the skill of the player, a Rook or Bishop might acquire a victory--or it might be summarily countered.
Recall, for each side, there is only one King. There is only one Queen. There are two Knights--perhaps evoking the notion that Rollo will be beaten or opt to switch sides. There are two Rooks/Bishops--Mao was beaten once, only to return. He had the King in check but a Pawn bested him.
I really don't like Code Geass enough to put this sort of thought into it, but shit like this comes naturally, for better or worse.