Sinthea "Sin" Shmidt is the daughter of the Red Skull. Seeking a male heir, the Red Skull fathered a daughter with a washerwoman. The woman died in childbirth, and the Red Skull almost killed the child, angry that it was a girl and not the boy he expected. One of his followers, Susan Scarbo, convinced him not to, telling him she would raise the girl herself as her nanny. The Skull agreed and left the girl (now named Sinthea) to be raised by Scarbo, who indoctrinated her with the Skull's views as she grew up. The Skull returned when Sinthea was a child and put her in a special machine that accelerated her aging process until she was an adult and gave her superhuman powers.
Afterwards, as Mother Superior, Sinthea became the leader of a group called the Sisters of Sin; young orphan girls who were accelerated into adulthood and given powers by the Red Skull after being indoctrinated by Sinthea. The Sisters of Sin would have many run-ins with the Red Skull's nemesis Captain America before being de-aged when they entered a chamber designed to reverse the Skull's aging process to assault Captain America - who had suffered through the Skull's process and had become elderly - while he was using it to return himself to normal, and they were reverted to children at the same time Captain America was restored (she would later claim she was de-aged to the wrong age - but whether this is true, and in which direction, is unclear).
Later, Scarbo (now calling herself Mother Night), reformed the Sisters of Sin and became their new leader, while the de-aged Sinthea took the name Sister Sin.
Sinthea was later captured by S.H.I.E.L.D. and taken to their re-education facility, where they attempted to reprogram her to be a "normal" American girl and gave her false memories to that effect. Later, the Red Skull was assassinated by the Winter Soldier under the orders of Aleksander Lukin, and one of the Skull's henchmen, Crossbones, broke into the facility and kidnapped Sinthea. Crossbones tortured Sinthea to break S.H.I.E.L.D's conditioning. After he succeeded, she entered into a relationship with him, and - with Sinthea now calling herself simply Sin - the two went on a killing spree. They later reunited with the Skull, now living inside the mind of General Lukin.
As the first part of the Skull's Master Plan, Sin disguised herself as a nurse after the Civil War while Crossbones sniped Captain America at the courthouse, even though it meant obeying her father and abandoning Crossbones to his fate. Sin then revealed to Sharon Carter that she was the one who had killed Cap.
While the Red Skull had control over Steve Rogers' body, Sin fought alongside Crossbones against the Avengers' forces. After Steve Rogers eventually regained control of his body, Sin was knocked unconscious. After the battle, and due to a subsequent explosion, Sin's face was left severely disfigured and scarred, leaving her a "red skull" appearance. Upon hearing the news, Norman Osborn quipped, 'Like father like daughter'.
Sin was institutionalized at the Kurtzburg Institute for the Criminally Insane People. During a breakout/riot, she was approached by Helmut Zemo, who asks her to tell him how to kill Bucky Barnes. More recently, Sin was liberated from the asylum by Master Man who has proclaimed both his allegiance and affection to Sinthea as the new Red Skull: the heir to her (believed deceased) father's legacy. 
After taking the Black Widow and Sam Wilson hostage, Sin sent one of her henchmen to disrupt the trial of Captain America and sent a demand: Bucky Barnes. After escaping with help from the manipulative Doctor Faustus, James confronted Sin and Master Man and brashly attempted to foil her escape. After the escape, Sin allied herself once more with Baron Zemo to retrieve the Book of the Skull, with which she would discover the whereabouts of a hammer her father summoned way back in 1942. After successfully retrieving the book and explaining its origins to Zemo, Sin betrayed him and went on her own to retrieve the hammer for herself.
Sin and her henchmen went to the stronghold of the Thule Society housing the hammer, invaded it and killed everyone who crossed her path. After finding the hammer and being deemed worthy by it, Sin, now as Skadi, went to set her new master, the Serpent, free. At her father's order, Skadi led an assault on Washington D.C., during which, she almost killed Bucky Barnes. Skadi soon reached New York, where she faced off against Steve Rogers, who had returned as Captain America. There, she summoned the Serpent. When the Avengers tried to take him down, he broke Rogers' shield and knocked out the team. By her father's side, Skadi and the other Worthy went to the fallen Asgard, in Broxton, Oklahoma, where they battled the Avengers once more. Sin was taken down by Rogers, who was wielding the mystic Mjolnir. After the Serpent was killed by Thor, Sin's hammer was taken from her by Odin, as well as the hammers of the other Worthy.
Search for Power
Suddenly, Sin woke up in a hideout where strange people said they freed her from custody and they would help to find her hammer again.
All-New Captain America
While on a mission to neutralize a new Hydra base in Ecuador, the new Captain America Sam Wilson was teleported along with his comrade, Nomad, from there to Helmut Zemo's palace in Bagalia through the Infinite Elevator. There, several super-agents of the new Hydra were expecting Cap and Nomad, with Sin being one of them.
After Zemo killed Ian, Sam, with an undercover Misty Knight, that helped him before in a fight against Crossbones, went to the Hydra's castle, only to be welcomed by Sin herself. There, the Red Skull's daughter faced Wilson while enjoying a reconstitution of the World War II through holographic technology. Sin started to mock Sam, saying that if it wasn't for her father, Sam wouldn't have become a hero, but only a low-level gangster by the name of "Snap Wilson". Sam, offended by her words, tried to attack her, only to discover that the Sisters of Sin were holding the members of his family, such as his sister and his niece, to ransom. If Sam attacked Sinthea, she'd order to her sisters to kill his family instantly. Sin made Sam give his shield and his wings to her, then ordered him to commit suicide by jumping off a cliff.
Soon after, she started a video conference with Crossbones, Baron Zemo and Taskmaster, saying that the plan to defame Wilson was successful and, after saying that she was better than her father, that she was the real and only Red Skull, she activated Zemo's plan: to spread worldwide the toxic blood of an Inhuman named Lucas, capable of sterilizing human beings, by launching a bomb into Earth's atmosphere. Zemo's plan was to create a new perfect world ruled by Hydra, as its agents would be the only fertile humans after the great leveling, thanks to the Inhuman boy's blood. It was then that Sin was attacked by Sam, who was rescued by his pet Redwing. Sin seemingly met her fate when the bomb containing Lucas' blood was destroyed by Cap.
Allying herself with the telepathic clone of her late father, Sin proposed him to create a new Hydra and use its agents to create deadly machines and infiltrate spy agencies, much to his dismay. Suddenly, a child appeared in front of them, and used her powers to fix Sin's face, returning it to how it was before the explosion that disfigured it. The girl then proceeded to explain she was made of a shard of a Cosmic Cube that once belonged to the Red Skull. Taking advantage of the situation, the Red Skull devised a new plan for his new Hydra.
Sin helped her father retrieve a black box, as well as all his gold, from his safe in a bank in Bagalia, being nearly caught by the Avengers Unity Squad in the process if not by the Red Skull, who used his telepathic abilities to disguise himself as Gambit, and to cloak Sin and the contents of the safe from them.
Sin and the Red Skull hid out in a secret underground room in the Avengers Mansion, which was no longer used by the Avengers and had been converted into a tourist attraction. The pair was nearly discovered by Quicksilver and Deadpool of the Avengers Unity Division, but the Red Skull used his telepathy to convince the heroes that the room was empty.
Sin later took her father to Pleasant Hill, a prison disguised as a small town where the realities of the criminals would be rewritten to turn them into powerless and common people, during Zemo's insurrection in order to the Red Skull to use Kobik to rewrite Steve Rogers' past, turning him into a loyal agent of Hydra at his service.
Being trained by her father, she is an expert hand to hand combatant and martial artist. She is also highly proficient in firearms and explosives. As the Red Skull's child, Sin also has a high level of intellect.
- 120 appearance(s) of Sinthea Shmidt (Earth-616)
- 10 minor appearance(s) of Sinthea Shmidt (Earth-616)
- 11 mention(s) of Sinthea Shmidt (Earth-616)
- 56 image(s) of Sinthea Shmidt (Earth-616)
- 13 quotation(s) by or about Sinthea Shmidt (Earth-616)
- 2 item(s) used/owned by Sinthea Shmidt (Earth-616)