The sniper.

• It moves like a king.
• It replaces one of the pawns on the board.
• If in any direction it has no more than one allied piece directly in front of it (cover), then at least (two? three?) unoccupied spaces between that ally and an enemy piece, it can take aim at an enemy piece.
• The next turn, if those conditions are still true, it can fire, capturing the target piece.
• An enemy piece adjacent to the target piece that could move into the line of fire can be sacrificed to take the bullet instead, though.