Defender icon is an item worn in the cape slot while playing Barbarian Assault. It shows that the player is a defender indicated by the blue circle around them with the defender 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 Defender role, which can be done via Commander Connad or choosing to do so immediately after a game, allows players to have an increased luring range when luring Penance Runners.