Maridel is the main villainess of The Mirror, a story from Adventures Into Weird World #8 (released in July 1952). She was a malevolent apparition who resided in the mirror of Christine Fox, with Christine sacrificing people to Maridel in exchange for increased years of life.
After Christine's stepson, Charles Fox, Jr, had a nightmare about Christine sacrificing his father to Maridel, he attempted to run away from home and was brought back by detective Jim Brady. To assuage Charles' fears by showing him the mirror, only to be shocked when Maridel's spirit appeared in the mirror at Charles uttering her name.
Like with his father, Charles, Jr. was entranced by the evil Maridel to enter the mirror, with Jim's attempts to save Charles by shooting the mirror failing to have any effect. Jim was subsequently lured into the mirror by Maridel, who granted Christine two more years of life before disappearing.