Margles is a character in Adventure Time, who seemed to be in a close relationship with Magic Man. It is never explained who she was, or what happened to her. The only evidence of her is a framed photo of her and Magic Man in his house. Abe Lincoln mentions that she and Magic Man spent a night together on Olympus Mons (a dead volcano on Mars), after which he started being a jerk. The promotional art for "Sons of Mars" suggests that an accident occurred during their journey as it shows a woman falling off of a mountain with Magic Man reaching after her, in tears.
From what is seen in the photo, Margles is a green Martian woman with medium length brown/orange hair. She wears a purple shirt that is similar to Magic Man's clothing. She is also visibly shorter than Magic Man in the photo, though she may be sitting or leaning as she is giving Magic Man a hug in this photo.