Spoiler warning for Supergirl Season 4: Plot and/or ending details follow...
Red Daughter (Russian: Красная Дочь) (June 18, 2018–), originally codenamed Snowbird,
is a copy of Supergirl who came into existence when Kara went back in time and utilized the Black Kryptonite to defeat Reign, materializing after a torrent of dark magic hit Siberia. She resides in Kaznia and was working with its soldiers and Lex Luthor in a plot to defeat the USA and resurrect the glory days of the Soviet Union. While undercover in the U.S., she used the name Linda Lee. After realizing Kara was right about Americans and that Lex was using her, Red Daughter sacrificed herself to protect her, taking a Kryptonite blast from Lex at Kara's place and merging back into her.
On June 18, 2018, as Kara Danvers used some Harun-El to transport herself, Samantha Arias, and Reign to Juru to defeat the Dark Kryptonian, the stone released a torrent of dark magic, which soon ventured into the Siberian Border and materialized into a copy of Supergirl. The copy, completely naked except for a blanket she was clad in, ventured forth through the snow and eventually approached a group of soldiers at the border. The soldiers took her to one of their huts, gave her a warm coat and a fur cap in order to cover herself as well as a cup of hot tea to warm up; then one of them tried to molest her by calling her "stray dog" and fondling her, but she grabbed his hand breaking its bones. At that exact moment the Minister of Defense arrived, commenting her likeness with Supergirl and interrogating her, however after a few questions, he realized that she was in a blank state with no memory at all except for the name "Alex". The Minister, eagerly decided to take the opportunity and train her to be a weapon for Kaznia's Military, nicknaming her "Snowbird" and giving her a copy of "Сбежавший Динозавр" in order to learn to write, read and at least some basic words. Some days later, Snowbird was escorted into the woods in order to be exposed at the sun and learn the source of her powers, managing to hovering into the air for the first time, however as her super hearing emerged, she became overwhelmed by the sounds and also developed heat vision randomly striking several Kaznia soldiers before the Minister of Defense ordered to restrict her with taser sticks.
Snowbird was then confined into a cell and guarded by the same soldier who tried to molest her some days prior. One night, due to her powers, she heard a young boy scream for help and escaped her confinment to reach the cabin in the woods where the boy, Mikhail, lived, saving him from some thieves and then hugging him and curling into fetal position in a corner of his house. There, after a few hours, she was found by Lex Luthor, who was called by the Minister of Defense in order to help Kaznia's Army to handle her.
Relying on her only memory, the name "Alex", Lex convinced Snowbird to be "Alex" and that they were very close friends before of her memory loss. He got close to her and asked her if she would like to learn how to control her strength, then having a first training session with her, and beginning to instruct, promising her to explain everything regarding her origins and sending her books and other material before departing due to his seventy two free hours away from Stryker's Island Penitentiary having nearly expired.
Later Snowbird started an intensive military and physical training. In one occasion, while she was punching through the rocky walls in a secret mine in the mountains of Southern Kaznia as part of her exercises, she caused an earthquake which created landslides above and destroyed part of a railroad track. After Supergirl saved a Kaznian train from being derailed, the copy continued her work under the watchful eye of the soldiers. She also read the books that Lex sent to her, learning English as well as several notions of politic and economy from Karl Marx, however the book she appreciated the most was The Great Gatsby, falling for the glamorous romance of the novel.
Some weeks later, Lex came back to her in Kaznia, to control her training and continue to teach her English, during a game of chess, Lex told Snowbird to consider her very similar to his sister, who, however, had disappointed and abandoned him; after which he scolded her for appreciating The Great Gatsby stating that he sent it to her to instruct her on how capitalism makes men weak and that the only reason he has money is to fight them. Before departing, he ordered her to read again the book while keeping this in mind. They also went to pay a brief visit to Mikhail in his cabin.
When the U.S.-based anti-alien hate group known as the Children of Liberty released Kryptonite into the Earth's atmosphere, Lex placed her into a special confinement cube to protect her and pierced her ears to make her more similar to Supergirl. Lex decided to spend the time necessary for the cloud to clear up to tell Snowbird that she was one of the last Children of Krypton and that her task was to start a period of peace on Earth by bringing together all the peoples and fighting the champions of corrupt governments: Superman and Supergirl. He also told her that Supergirl was her sister and robbed her of her place in the World. A couple of days later, after L-Corp nanites were used to purify the atmosphere of Kryptonite particles, she was released and resumed her training.
Visiting National City:
After a month, Lex took Snowbird to National City under the alias of "Linda Lee", in order to let her see how dissolute are the lives of the people that Supergirl protected. While posing as a waitress into a bar, "Linda" was disgusted to see some ArmeTek officials spending that much in drinks while Kaznian people are starving, as well as objectifying her by calling her "doll". Lex also brought "Linda" to see Kara Danvers' apartment so that she learned her way of thinking and developed disgust for her counterpart's luxurious lifestyle. While there, Lex and "Linda" were almost caught by Alex Danvers, but the copy managed to deceive her posing as Kara and telling she returned earlier from her trip in Smallville. Then, despite Lex leaving the apartment, "Linda" chose to stay a little longer and began to read Kara's journal, learning of her friends and caring attitude, so she decide to approach Lena Luthor at CatCo pretending to be Kara, however Eve Teschmacher confronted her in an elevator, sent Lena away with an excuse and scolded her behavior, ordering her to went back to Kaznia immediately.
Few hours later, in order to solve the fact that Snowbird "has a conscience", which could be a problem for his project, as well as to strengthen her resentment towards the Americans, Lex ordered Eve and Otis to stage the bombardment of the house of Mikhail, by an American aircraft carrier.
However, as she discovers it, she attacked the aircraft carrier, ignoring Lex's advice not to do it, and, after he stopped her and staged her attack at the platform to seem like an accident, he scolded her for having risked to compromise all their work due to a moment of weakness, after Snowbird confronted him about everything he wasn't telling her, Lex simply replied she had "disappointed" him.
Illness and rise of Red Daughter:
Months later, while continuing and hardening her training, Snowbird fell sick due to some sort of disease, since the exposure at Green Kryptonite having severely weakened her immune system, and eventually became comatose. Despite the attempt to save her via electroshock at full charge (which was so powerful to went through overseas and affected some drugs, giving superpowers to it's users), she ramined unconscious so, Lex decided to enter a radioactive chamber in order to developed an inoperable cancer and asking for his sister's help to get an Harun-El drug capable to cure himself and thus Snowbird. So, after having set his plan into motion, escape from house arrest and beated Supergirl, he went to Kaznia and gave Snowbird a blood transfusion in order to restore her health, apologizing with her for having leave, giving her a suit and calling her his legacy: his "Red Daughter"
Attacking the White House:
The following day, she went to Washington, D.C. alongside Eve and was instructed regarding her role during that evening's operation while inspecting the sewers under the White House; after Eve treated her like she doesn't understand English, Red Daughter protested stating that she knew it better than Eve herself, causing the latter to smirk and comment that she looked exactly like Supergirl. Then, after the official repeal of the Alien Amnesty Act, while Eve distracted the real Supergirl by enclosing her in a Lexosuit alimented via Kryptonite, Red Daughter, posing as her, killed all the congressmen in the Oval Office and escape, prompting President Baker to declare the "Girl of Steel" public enemy number one.
Later she settled in National City into a small apartment in front of Kara's and furnished it in the exact same way, though it is unclear whether it was out of envy or to mock her.
Battle against Supergirl:
When Supergirl is captured because of Baker working for Lex Luthor, Red Daughter tries to kill her after weakening her with a piece of Kryptonite, but her template managed to escape due to her higher resilience to it. Then, Red Daughter went to Midvale while Eliza Danvers was home and made her call Alex via phone, knowing Kara was with her sister at the Danvers house.
When Supergirl came to her, she threatened to kill Eliza by telling her that her relationships and possessions made her weak, while claiming she was strong because she served the collective rather the individual, to which Kara asked her about Mikhail (having discovered how much she cared for him by reading her journal) however Red Daughter answered that "the Americans had killed him" and they started to fight. During their battle, Alex remembered again that Kara is Supergirl and Red Daughter revealed that she had purple-lightning-emission powers. Red Daughter puts Kara into a near death coma and then left the battlefield after hearing the heart of her template that slowly stopped beating, unaware of the fact that Kara had managed to seep yellow sun radiation from the grass around her to restore herself.
Betrayed from her only friend and ally, Red Daughter was murdered during the Kaznian Invasion of the United States as a way to further frame Supergirl and make Lex seem as a hero, accomplishing his plan. However, all of this was simply staged by Lex as Red Daughter was actually stunned and imprisoned in Shelley Island to power a new Claymore satellite with the energy drawn from her powers in order to use it to destroy Superman and Argo City.
Helping Supergirl and sacrifice:
Rebirth and the fallen Supergirl:
The story of the Red Daughter did not end with her sacrfice.
Harun-El mutation: As a product of the Kryptonian artifact Harun-El, Red Daughter was created as a materialized alternate persona of Kara Zor-El. After suffering from Kryptonite poisoning weakened her immune system, Red Daughter was given a blood transfusion from self-mutated Lex Luthor, introducing his Harun-El anti-cancer cure to her immune system
Expert Tactician: Kara has been trained in military tactics, being able to command a team of operatives, often using diversions to accomplish her goals.
Power bestowal: While having a shock and in an attempt to save her, she was electroshocked at full charge, and the shock was so powerful, it went through overseas and affected some drugs, giving the user of the drugs strange abilities.
Expert Pilot: She is an accomplished pilot, able to fly a Federation shuttlecraft, an X-Wing Starfigther, or a Spitfire even when a fierce battle is going on.
Harun-El Blast: Red Daughter had the ability to launch a blast of energy from the Harun-El inside of her in order to shock opponents.
Singing: She is a talented singer like her counterpart Kara Danvers, able to pass as a lounge singer.
Kryptonian physiology: Normally, like all Kryptonians, Red Daughter's capabilities were no greater than a normal human of her physical conditioning. However, once charged by the energy spectrum of a yellow sun, she becomes able to perform various inhuman feats.
Dancing: like her counterpart Kara Danvers, she also is a talented dancer, particularly in ballroom and tap.
Genius-level intellect/Economic acumen/Political acumen: Red Daughter was very intelligent, thanks to both her natural intelligence and to Lex Luthor's training. She was an avid reader of literature, a student of economics, politics and languages. Red Daughter studied books on economics, namely those written by Karl Marx, she has also studied books on politics.
Solar energy absorption: While her powers are dependent on the energy spectrum from a yellow sun, her body was able to constantly and passively absorb such energy while exposed to it, essentially keeping her reserves fully charged near-constantly. Direct exposure to the light of a yellow sun would also accelerate her recovery from any injuries she does manage to sustain.
Eidetic memory: Red Daughter has an eidetic memory, having been able to learn complex concept at an extremely fast rate.
Accelerated healing factor: Red Daughter's metabolism was tremendously enhanced by solar energy, allowing accelerated healing abilities and burning calories at a superhuman rate, making her practically immune to weight gain.
Master of deception/Manipulator: Red Daughter was extremely talented in the art of deception; as she was able to pose as an American waitress and also managed to successfully portray herself as Kara Danvers to both Alex Danvers and Lena Luthor respectively, with little-to-no effort, as neither Alex or Lena suspected that she was not the real Kara.
Self-sustenance: Due to the effects of a yellow sun, Red Daughter's physical needs were greatly reduced or completely removed.
Atmospheric adaption: While she does required oxygen, her physicality allowed her to survive while inhaling more harsh forms of it, or even no oxygen at all.
Contaminant immunity: Red Daughter had an immunity to all forms of disease and contaminants on Earth.
Bilingual: Red Daughter was capable of fluently speaking Russian and English. She spoke these languages at a native level, effectively switching between them with ease.
Flight: Red Daughter is able to manipulate her own gravitational field to generate thrust and propel herself through the air, often at hyper-sonic speeds, much faster than she could travel by foot. As such, she is able to levitate and fly past Earth's atmosphere and near the planet's orbit.
Levitation: She can levitate using her flight abilities.
Heat vision: By concentrating every solar energy reserve she has in her body, Red Daughter could emit blue energy beams of variable intensity and temperature from her eyes.
Expert hand-to-hand combatant/Martial artist: Having receiving several months of intensive training from the Kaznian army, Red Daughter is a highly capable hand-to-hand combatant and martial artist.
Invulnerability: While under the affects of the yellow sun, Red Daughter was essentially invulnerable to all Earthly weapons, with bullets simply ricocheting whenever they come into contact with her skin.
Extreme heat resistance: Red Daughter felt no pain when exposed to extreme heat.
Elemental resistance: She feelt no pain when exposed to harmful elements like rocks, fire, electricity and ice.
Extended longevity: As a Kryptonian, Red Daughter's life span was considerably longer than a normal human and likewise caused her to age much slower.
Super breath: While under the affects of the yellow sun, Red Daughter was able to exhale powerful gusts of air from her mouth which are similar to force winds. She could also cause the temperature of her breath to drop, therefore able to freeze nearly anything.
Super hearing: While under the affects of the yellow sun, Red Daughter had super-sensitive ears that could perfectly pick up sounds from miles away and even through structures.
Super speed: Red Daughter possessed the ability to move at hyper-sonic speeds, both through flight and on foot.
Accelerated perception: While using her super speed, Red Daughter saw everything much slower; allowing her to move with precision and accuracy within very fast moments.
Super strength: Red Daughter's strength was enhanced under a yellow sun, enough to easily kill a normal human if she were to attack them directly.
Super jumping/leaping: Red Daughter was able to jump several feet off the ground and leap several stories in a single bound without having to fly.
Super stamina: Red Daughter could run, fight or fly for long periods of time, without getting tired.
Super reflexes: Red Daughter's reflexes were so fast, she can respond to attacks within microseconds.