Attacker icon is an item worn in the cape slot while playing Barbarian Assault. It shows that the player is an attacker indicated by the red circle around them with the attacker symbol. It cannot be removed or interacted with in any way besides examining it. When a game of Barbarian Assault finishes, the icon is removed.
Having a higher level in the Attacker role, which can be done via Commander Connad or choosing to do so immediately after a game, allows players to deal bonus damage to penance monsters.