"Long live the queen." - Abaddon
Abbadon is a demon, which means that her true form looks like black smoke. In the show she mostly possesses a tall woman with long, red hair. She also ironically wears a "The devil made me do it"-shirt.
Abaddon is merciless. She takes what she wants and she doesn't care about others. She's not afraid to kill to achieve her goals, quite the opposite, she enjoys inflicting pain on others.