The spy. It takes the form of one of your pawns but only you know which pawn is the spy.

If the spy kills a piece the way a regular pawn does, it ceases being a spy and reverts to bring a regular pawn.

If you position the spy on one side of an enemy piece with another of your pawns on the other side of it, you can recruit the piece as a pawn on your side (regardless of what class it was)

Kings and Queens are immune to spies.

When a spy is killed, any pieces it recruited are reverted to enemy pawns.