While on his Isle of Exiles stronghold, the Red Skull chose a peasant washerwoman reminiscent of his mother to bear his heir. The woman died giving birth to a daughter. Furious, having wanted a son, the Skull spat in the dead woman's face, then took the newborn to throw her into the sea. Convinced by his underling Susan Scarbo to spare the child, the Skull ordered Scarbo to be her "night mother" and raise her in misery, anger and hate at Skull-House, an old Catholic monastery, only seeing her himself five minutes a week.
The young Sinthia Shmidt was a brutal beast of a girl; Susan Scarbo's prize pupil. Learning he was dying and needing an instant heir, the Skull used his Deus Machina to artificially age Sinthia into adulthood, mentally indoctrinate her and empower her as Mother Superior. The Skull then aged and empowered four orphan girls as Sinthia's worshippers, the Sisters of Sin: Sisters Agony, Death, Dream and Pleasure.
The Death of the Red Skull
Still hesitant about a woman heir, the Skull had Sinthia recruit and train Baron Helmut Zemo, then played them against each other. Aided by Sister Pleasure and a gullible Nomad, the Skull accelerated Captain America's aging. Sinthia and Zemo kidnapped Cap's pacifist friend Dave Cox, turning him into the violent Devil-Slayer, then abducted Cap's childhood friend Arnie Roth, drawing Cap and Nomad to Skull-House. The Sisters also abducted Cap's girlfriend Bernie Rosenthal and the Falcon.
Sinthia battled Zemo for right of succession, apparently killing him, but the Skull rejected her then died of a heart attack fighting Captain America. Unbeknownst to the others, Nazi geneticist Arnim Zola transferred the Skull's consciousness into a Cap clone. After callously burning the Skull's body, Sinthia and the Sisters attacked Avengers Mansion. Entering the room where the Deus Machina was rejuvenating Cap, they were de-aged to teenage status.
Sisters of Hate
Taken in by Susan Scarbo (now Mother Night), the teenage Sisters became sadistic "counselors" in a youth camp designed to indoctrinate teen runaways into hate. Now calling herself Sin (with Death, Dream, Pain and Pleasure becoming Torso, Hoodwink, Slash and Raunch), Sinthia recruited "Roger Stevens," in reality Captain America transmuted into a teenager by the Eternal Sersi to better infiltrate the camp. Cap broke up the camp but the Sisters escaped, and Sinthia later became one of the reborn Skull's division chiefs.
Sin was captured and brainwashed by SHIELD to believe she was Erica Holstein and kept at the Secure Government Re-Education Facility outside Reno, Nevada. After Kronas Corporation head Aleksander Lukin had the Skull murdered, Skull's lieutenant Crossbones freed Sin and tortured her until she recalled her true past.
Going on a Midwest murder spree, the two stumbled upon and took over a hidden RAID facility. Discovering Lukin's involvement in the Skull's death, they plotted to kill him until they learned the Skull had inserted his consciousness into Lukin's mind. Reunited with her father, Sin became one of his top agents.
The Death of Captain America
Sin and Crossbones were a part of the assassination of Captain America following the Civil War. Crossbones, who shot Captain America from a sniper, was caught and beaten by an angered Winter Soldier and Falcon. Sin managed to escape, while Crossbones was caught and imprisoned for the assassination.
When both women had recovered Sharon took Sin hostage and attempted to escape from the Skull's fortress. Sin realized that Sharon is still slowed by the sedatives and stabbed her resulting in her miscarriage of Steve Rogers' child. The Skull was infuriated and gave her one chance to redeem herself, she was sent to the presidential debate to pretend to attempt to assassinate Senator Wright to make him look like a hero but Captain America intervened. She was knocked unconscious by the explosion of her rocket propelled grenade and taken into custody.
Reviving Steve Rogers
Sin was instrumental in her father's master plan to put himself into the body of Steve Rogers, which was financially backed by Norman Osborn. The Avengers intervened and Sin was caught in the explosion of her Father's robotic armored body. Her father appeared to die in the explosion, but Sinthia was merely injured. At the time of the explosion Sin was wearing a costume and mask reminiscent of the Red Skull and now her face is scarred to resemble her fathers, thus making her the new Red Skull.
Following the aftermath of the "Trial of Captain America", Baron Zemo freed Sin from her imprisonment and the two of them sought out the Book of the Skull in Egypt. The story of her father's greatest failure in trying to harness the powers of an ancient fear god for the Nazi's during the war. Sin takes the notes needed to find the hammer that was left guarded after the war ended and shoots Helmut's teleporter and steals his ship. Left alive in the desert, Zemo tells Sin that he hopes she succeeds where her father failed because today, she made a new enemy as he left to find his way out of the desert. She journey's to the arctic where she fights through her father's men who have been guarding the hammer that fell to Earth during the war. As she approaches it, her father's men tell her that no one has been able to lift it since it first fell but Sin tells them that she saw it in a dream and that she saw Captain America (James Barnes) dead at her feet. She touches the hammer and is transformed into Skadi the goddess of fear. She quickly flew to the Pacific Ocean where she battled a group of Norse dragons that were protecting her father, the original high father of Asgard and the god of fear. The two quickly left their prison as her father summon more hammers to pick "the worthy" to bring back their lost brethren.
Marvel Avengers Alliance
Sin is a boss in several missions in the Game. During the "Fear Itself" storyline she was featured as Skadi.
Her "Sin" bio says: Desiring to have an heir, the Red Skull indoctrinated and trained his daughter mercilessly, hammering her into the shape he wished. He got what he wanted: Sinthia Schmidt is just as brutal and immoral as her father.
Her "Skadi" bio says: While seeking an ancient power first discovered by her father, Sin came across a mysterious hammer. Upon touching it, she was transformed into Skadi, Herald of the Serpent.
Marvel: War of Heroes
Sin appears in several cards in the mobile card game Marvel: War of Heroes. Her cards are:
- [The Worthy] Sin
- [Skadi] Sin
- [Never Surrender] Sin
- [Sister] Sin