Supergirl

By 1942, Superman had become a phenomenon, so National (DC) printed three small black-and-white comics called "ashcan" comics titled Superwoman, Superboy and Supergirl. This was common in the day in order to secure the trademark on a name or title.

None of these ashcans actually depicted a super-hero. It took ten years before the concept of "Superboy" took hold, and yet another ten for DC to try a proper "Supergirl."

Lois Lane was called "Superwoman" several times between 1943 and 1951. Two other one-off characters were also described as "super-girls."

Silver Age

The classic, original Supergirl was, simply, Superman's long lost cousin from Krypton. She was introduced in Action Comics #252 (May 1959), where she was set up with the secret identity of "Linda Lee " in an orphanage in the town of Midvale.

The character was popular but Supergirl lived rather in the shadow of Superman for most of her existence. She occupied the backup feature in Action Comics for ten years befor moving to Adventure Comics (a slot she took over from her friends, the Legion of Super-Heroes). In 1972, the Maid of Might was finally awarded a solo title but it fared poorly and was canceled after ony 10 issues.

Supergirl was sent back into the anthology world, anchoring Superman Family from 1977–1982. After a second failed series (Daring New Adventures of Supergirl, 1982–84) Kara Zor-El's fate was sealed.

Supergirl was never welcomed into the Teen Titans or the Justice League — or despite her age never called "Superwoman." But she had a long history as a member of the Legion of Super-Heroes. Had she relocated permanently to the 30th century, she might have escaped her death sentence as a casualty of the Crisis on Infinite Earths.

Post-Crisis

In "post-Crisis continuity," the DC universe was rebooted and the decision had been made that Superman should be the sole survivor of Krypton. Kara Zor-El was totally absent from the new timeline. Instead, a pseudo-Supergirl, called "Matrix," was created. This hero was not from Krypton but a protoplasmic shape-changer from a Pocket Universe. Matrix looked like the Supergirl of old and became a fixture in Metropolis.

Crisis on Infinite Earths also changed the history of the Legion of Super-Heroes. Neither Superboy nor Supergirl had been a member of the group, so a 'replacement Supergirl' was invented. She was a Daxamite called Laurel Gand.

Infinite Crisis

When the Kryptonian Kara was reintroduced to the DC universe in 2004 (post-Infinite Crisis) she arrived on Earth for the "first time." This Kara was totally distinct from her Silver Age predecessor but some attempts were made to retcon her back into original Legion membership.

The DC universe was rebooted again in 2011 as the New 52. The Supergirl of this era is totally distinct.

The Post-Crisis Supergirl (II): Matrix

For many years, editorial edicts forbade the reintroduction of Kara Zor-El, preferring that Kal-El exist as the "sole survivor" of Krypton. One notable exception is a short story from Christmas with the Super-Heroes #2 (1988), in which Deadman receives some advice from a woman named Kara. She encouraged him to do good deeds, "even if no one remembers we ever existed."

Most of Supergirl's original history was invalidated by Crisis on Infinite Earths and the reconstruction of continuity therafter.

John Byrne created a different kind of Supergirl in Superman vol. 2 #16 (1988). This Supergirl was from a so-called "Pocket Universe," and was made from a protoplasmic matrix created by Lex Luthor. (This might have been an allusion to the "protoplasmic lifeform" that was created and destroyed in Adventure Comics #271, Apr. 1960.) This "Matrix" was created by Lex in his attempt to fight three Kryptonian villains (General Zod, Quex-Ul, and Zaora) who had escaped from the Phantom Zone.

At first Lex fashioned the Matrix's likeness to that of Lana Lang. He then outfitted her with a costume reminiscent of Superboy's, and dubbed her Supergirl (II). Lex learned from Pete Ross that there was another universe and a Superman there, so he sent Supergirl to enlist his help. (Adventures of Superman #444)

Matrix/Supergirl arrived in Superman's past and became buried in Antarctica until she was unearthed. (Superman vol. 2 #16) She suffered from partial amnesia and flew to Smallville where she met Lana Lang and the Kents, then set out after Superman. (Adventures of Superman #442, Superman vol. 2 #20)

Superman quickly deduced that this Supergirl was from the Pocket Universe, a revelation which caused her memory to return. She escorted him back to her universe, where Superman met the alternate Luthor and learned of their peril. (Superman vol. 2 #21, Adventures of Superman #444) The Kryptonian villains wiped out all life on that Earth, and Supergirl was blasted back into her protoplasmic state. Superman made the fateful decision to execute the criminals by using Gold Kryptonite to strip their powers, then Green Kryptonite to kill them. He returned to his own universe with the badly wounded Supergirl, leaving her in the care of his parents. (Superman vol. 2 #22)

Supergirl III

Eventually, this Supergirl became bonded to the body of a human named Linda Danvers. The mystical nature of that event made their merger permanent and this Supergirl (III) retained her super-powers, including flight, invulnerability, telekinesis. She lost the ability to become invisible and over time her ability to change shape became limited switching between 'Linda' and 'Supergirl.' (Supergirl vol. 4 #1–3)

Danvers adventured as Supergirl for many years, acquiring additional powers as an "Earth angel." This Supergirl met (but did not join) the Reboot Legion in Supergirl Annual #2 (1997). After Infinite Crisis, the existence of Linda/Matrix was in question because the Pocket Universe ceased to exist.

After Infinite Crisis (2007), it was unclear whether the post-Crisis Supergirl was removed from continuity. However, Linda appeared as an angel in Reign in Hell #6 (2009). In that tale, Dr. Occult called Linda "Supergirl" and she responded "yes... no! It's a long story." (Peter David, the author of her most popular adventures, later launched a series called Fallen Angel, which teased a new character's connection to Supergirl; it was explicitly confirmed, and the character was sold to IDW.)

Other Post-Crisis Supergirls

Another Supergirl (IV) — from the future — was also introduced in Superman the 10 Cent Adventure #1 (2003). She was named Cir-El, the daughter of Clark Kent and Lois Lane. (Superman: The 10 Cent Adventure) Cir-El's alternate future timeline that was wiped from continuity even before the Infinite Crisis.

"Elseworlds"

In 1995's Superman/Aliens crossover (published in collaboration with Dark Horse Comics and 20th Century Fox, which owns the Alien franchise), Superman encountered a 16-year-old blond girl named Kara from a "space city" called Argo, whose people spoke the Kryptonian language and had architecture and technology similar to Krypton's. Later, Kara explained that Argo was not from Krypton, but rather from a planet called Odiline, which had adopted Kryptonian customs after a visit from the space ark of the Cleric (introduced in Action Comics Annual #2) millennia earlier. Superman and Kara eventually destroyed Argo to stop the deadly alien parasites that had already wiped out the rest of the city's population, and Superman returned to Earth believing Kara had died in the explosion that destroyed her city. In fact, she had made it to an escape pod and survived, although her ultimate fate was not revealed. As an inter-company crossover, it's not clear to what extent that story can be considered part of post-Crisis DC continuity.

Animated

In 1998, Superman: The Animated Series used a similar conceit (albeit without the Aliens) in the origin of the animated Supergirl. That version of Supergirl, also named Kara, had the same powers as her Silver Age counterpart, but was technically not Kryptonian. Instead, she was the last survivor of Krypton's "sister planet," Argos, which had been devastated by the explosion of Krypton. The animated Supergirl was not part of contemporary post-Crisis continuity, although she appeared in Superman Adventures and other comic books tying in with the animated series.

Powers

Matrix took a female form that was similar to Superman's but her powers were quite different. She was a shape-changer, one whose protoplasmic form eventually settled permanently into that form. She possessed telekinesis which enabled flight, and she used it to mimic the effects of super-strength and invulnerability.

Supergirl V: Infinite Crisis

Nearly 20 years after Crisis on Infinite Earths, the Infinite Crisis wrought a true, new Kara Zor-El. She was reintroduced to the Superman family in Superman/Batman #8 (May 2004). This Supergirl was a clean slate, with her own distinct history.

As in Silver Age continuity, Kara Zor-El was the cousin of the legendary hero, Superman (Kal-El). Both were born on Krypton, but Kara’s journey to Earth was quite different than his. Unlike her cousin, Kara grew to adolescence on Krypton and saw its destruction firsthand. After her uncle Jor-El sent the infant Kal-El to Earth, Kara’s father Zor-El planned the same thing for her.

Kara's home, Argo City, survived the destruction of Krypton. As the planet began to fall apart, Zor-El managed to surround Argo City using force field technology (which was recovered from Brainiac's attack on the city of Kandor). Argo drifted in space and ultimately crossed paths again with Brainiac, who destroyed the city and assimilated its inhabitants into his shrunken Kryptonian city of Kandor. (Supergirl vol. 5 #35, Action #869)

Kara’s spaceship to Earth took longer to arrive than Kal-El's. It was also encased inside kryptonite (toxic fragments of the doomed planet). When her ship finally fell to Earth, it was discovered by Batman in Gotham City. When Kara awoke, she met her cousin, Superman. (Superman/Batman #8)

In her first days on Earth, her mind was in turmoil. She could not fully remember her life on Krypton. Meanwhile, she trained in the use of her newfound powers, which were extraordinary under a yellow sun. Wonder Woman took Kara under her wing, and the Amazons of Themyscira taught Kara their ways and battle techniques. (#9) Supergirl's power soon drew the attention of the evil god Darkseid, who kidnapped her. (#10) She was quickly rescued by Superman, who gifted her with a new costume bearing the El family insignia, and title: Supergirl (II). (#12)

During the "Infintie Crisis," Supergirl eventually joined up with a group assembled by Donna Troy (formerly Wonder Girl), who ventured into space to investigate a cosmic rift developing there. (JLA #123, Infinite Crisis #1) This rift collapsed when its creator, Alexander Luthor was defeated. Supergirl was thrust, amnesiac, across the multiversal divide—to Earth-Prime in the 31st century. (Infinite Crisis #7, 52 #5)

Notes

Zor-El's first post-Crisis appearance was Superman/Batman #9; and Alura in Supergirl v.5 #16. When her origin was elaborated upon (in Supergirl vol. 5 #16, 2007), Zor-El was supposedly mad and had sent her to Earth to kill Kal-El. She believed he was possessed by malevolent phantoms. This was later explained/retconned away, as the effects of kryptonite poisoning on Kara's mind in Supergirl vol. 5 #35.

Joining the Legion of Earth-Prime (Threeboot)

After Kara Zor-El was reintroduced in 2004, some of her previous history with the Legion of Super-Heroes was retrofitted so that she could remain a member of the group. (This was during the "Retroboot" era of Legion continuity.) In fact, she became a member of two Legions, from two universes…

When Supergirl arrived in the 31st century on Earth-Prime, she believed that she was just dreaming. When she met the Legion’s Cosmic Boy, she told him that Legion didn’t really exist. Regardless, the inhabitants of Earth immediately recognized her as a legendary hero. (Supergirl and the Legion #16) Despite her disbelief, Supergirl was immediately on-hand to help where it was needed. Cosmic Boy also proposed that the Legion make Supergirl an honorary member. (They could not award her full membership because they had no spare flight ring; the rings were rare and expensive to make.) This situation was rectified by Brainiac 5, who offered her the ring of the recently deceased Dream Girl. (#17)

The Legionnaires soon feared that Supergirl would become a danger if she didn’t accept her situation as reality, so they subdued her with kryptonite and took her to the legnedary Kryptonian city of Kandor, which in the 31st century resided on the planet Rokyn. Kara was overwhelmed to learn that part of Krypton had survived. (#23) But according to Kandorian law, Kara would be required to either stay there forever, or leave forever. The Legionnaires advised her to return to Earth, which she did. (#24)

Kara had impressed her peers thoroughly, and when Cosmic Boy opened the Legion leader elections to thousands of Legionnaires at-large (#21), Supergirl won! Her first act as leader was to address the entire organization, and to offer Legion membership to the members of the rival Wanderers. (#31)

After an epic battle against the Dominion, Supergirl began in earnest to look for a way home. Brainiac 5 helped her, first by constructing the Chronexus which allowed her to view into the past. (Action Comics #850) He soon succeeded by teaming up with another hero, Evolvo-Lad, who modified the Chronexus to send send Supergirl home. (#35-36)

During her time on Earth-Prime, the Legionnaire Invisible Kid developed a serious crush on her, but he never expressed his feelings. (#22)  On parallel Earths, it is has been Brainiac 5 who is normally enamored with her. On Earth-Prime, Brainy’s affections had already been claimed by his teammate, the platinum bombshell Dream Girl.

Back in the 21st Century

In Kara's search for her identity, she tried unsuccessfully to join groups like the Outsiders (Supergirl vol. 5 #3, 11-12) and the Teen Titans. (#22) She made some good friends within those groups, though, with both Wonder Girl and Captain Boomerang. But an attempt to attend public school and assume a secret identity as Claire Conners failed as well. (#10)

Her relationship with her namesake, Power Girl, has also been tumultuous. After a rocky first meeting (#1) Supergirl and Power Girl undertook a mission to find Argo City. In her heart, Supergirl believed that Argo had survived the destruction of Krypton. Her search for the truth led them to a a city called Kandor—a society inspired by the original Kryptonian city. There they donned the guises of Flamebird and Nightwing and discovered that Kandor's rulers (Saturn Queen and Ultraman) were villains who had also survived the destructions of their universes. In the end, Kara made a deal with Saturn Queen to leave the city in exchange for information about Argo City.  (Supergirl vol. 5 #6-8) NOTES: Nightwing and Flamebird were aliases originally used by Superman and Jimmy Olsen during their trip to Kandor (Superman #158, 1963).

Kara's early days on Earth were dark days because, unbeknownst to her, she suffered from residual kryptonite poisoning. This affected her memories and judgment. She doubted her very nature, and worried that she'd been sent to Earth only to murder her cousin, Kal-El. (#5-7, 16) To protect Kal from this fate, Kara cut off all contact with him for some time. She was cured of this malady by none other than her own parents! Zor-El, Alura, and Kandor were discovered by Superman while battling Brainiac in space. (Action #868-869) Back on Earth, Kandor was enlarged, (#870) and Kara was reunited with her parents. (Superman: New Krypton Special #1)

Zor-El was murdered soon thereafter by Reactron. (Action #872) Kandor retreated into space, forming its own planet, New Krypton. In memory of her father, and to help support her mother better, Kara chose to become a member of the Science Guild on her "Guilding Day," a the traditional Kryptonian ceremony. (Supergirl #43)

Rejoining the Legion of Earth-0 (Again)

In the reconstructed multiverse of the post-Infinite Crisis era, Supergirl's interaction with the original Legion of Super-Heroes was restored, sort of. In the era of the "Legion Retroboot," her involvement was restored, but since the post-Crisis Kara was so different from the original, that history with the Legion was more convoluted.

Her first Legion-related encouner was with Brainiac 5's 21st century ancestor, Vril Dox (Brainiac 2). She aided him when he was on Earth. (R.E.B.E.L.S. vol. 2 #1-2)

Supergirl's memories about her time with the Earth-Prime Legion were a bit fuzzy when she met two familiar Legionnaires from Earth-0 — Karate Kid and Una. She helped these familiar heroes on their mission in the 21st century. (Supergirl vol. 5 #21-22)

Soon after that, Supergirl inadvertently arrived in the future, where she met the (Retroboot) Legion and decided to remain for months (this was after her cousin had joined). She was comforted by the Legionnaires and eventually joined their ranks. In one major case, she and Brainiac 5 teamed to stop the invasion of Satan Girl. They became attracted to one another and promised to meet again before she returned to the 21st century. Saturn Girl (as she had with her cousin, Kal) removed selected memories of Kara's visit to the future. (Supergirl vol. 5 Annual #2) Note: Unlike the original "Satan Girl" story (Adventure Comics #313Oct. 1963), the villain was not Kara herself.

After the DC universe was rebooted for "The New 52," the Supergirl of that continuity did not have a history Legion. Again.

Powers

Supergirl possesses the same powers as all Kryptonians under a yellow sun: super-strength, super-speed, invulnerability, x-ray vision, heat vision, and freezing breath.

She can survive unaided in space roughly two hours by holding her breath, but does require oxygen.

These powers can be negated primarily by red sunlight, kryptonite and magic.

Kingdom Come: Brainiac's Daughter

Kingdom Come was a 4-Issue limited series in 1996 created by Mark Waid and Alex Ross. They explored the future state of the DC Universe and created children and legacy heroes for many of DC's iconic heroes.

In both the Kingdom Come trading cards and the hardcover Revelations supplement, Brainiac's Daughter was described as "the living computer's [Brainiac's] human progeny, and ancestor of Brainiac 5." The unofficial name for this character is XTC, because she was Mark Waid's homage to the band of the same name. XTC recorded songs titled "That's Really Super, Supergirl" and "Brainiac's Daughter."

Because the character's costume was adorned with a big "S," some fans assumed that she was the daughter of Brainiac 5 and Supergirl. Mark Waid debunked that; it wasn't his intention, but he did write, "I'll go along with this idea."

In Kingdom Come, Supergirl herself had moved to the 30th century to be with the Legion.