Supergirl is Linda Danvers, but this is more than just a secret identity.
Before she met Matrix, Linda was an ordinary human teenager living in the small town of Leesburg, while the superheroine known as Supergirl was a shapeshifting artificial being with the memories of another world’s Lana Lang, endowed with superpowers both similar to and very different from Superman’s. Supergirl, also known as Matrix, had telekinesis, invisibility, and shapeshifting as her main powers.
The Search For Identity (SUPERGIRL #1-9)
A talented artist, Linda viewed Supergirl as her role model and frequently found herself sculpting the superheroine’s form. But an evil demon named Buzz had played upon Linda’s loss of faith in those around her by drawing her into his cult, in preparation for using her as a human sacrifice to an otherworldly creature known as Lord Chakat. Unbeknownst to Linda, Buzz’s plans for her went much deeper than that, and the ritual sacrifice was intended to draw in Supergirl. Supergirl managed to reach Linda before the sacrifice was completed, but not in time to save her life. With Supergirl filled with grief over her failure, and Linda feeling overwhelmed at how her life was ending before she could set right what had gone wrong, the two reached out to each other at the moment of death and Supergirl merged her protoplasmic form with that of Linda’s. Through a convergence of supernatural events, Linda Danvers and Supergirl were brought together in a merging of physical form and consciousness that brought new meaning to both their lives. Linda and Supergirl found what they had both been searching for within the other: a soul and a human identity for Supergirl and a sense of hope and purpose for Linda.
The Earth-born Angel of Fire (SUPERGIRL #10-50)
As Supergirl/Linda struggled to reconcile her two lives, two sets of memories, and two very different characters, she found herself manifesting new powers. Supergirl’s decision to sacrifice herself for Linda unexpectedly caused her to become an "Earth Born Angel". The Girl of Steel found herself with wings of flame, an ability to teleport, and the power to “judge” evildoers as the Angel of Fire.
Eventually it would be Linda’s personality that came to dominate, but not before Supergirl learned (in Supergirl #49) that Kara had been Linda’s “guardian angel” since childhood. Kara’s spirit had been with her all along, waiting for her destiny to find her.
The Earth-Born Angel is Lost (SUPERGIRL #50)
In issue #50 Supergirl fought alongside her fellow earth angels, two other joined beings, in the climactic battle to defeat the first vampire, the Carnivore. In defeating this powerful demonic being who had been her nemesis for fifty issues, Supergirl lost her Earth Angel aspect and attending angelic powers. Amazed that she had survived, but realizing that her “Supergirl” half had been separated from Linda Danvers, she found herself with reduced superpowers and thus began a quest to find the “missing” spirit of Supergirl/the Earth Angel. This ended her “Angel of Fire” phase. The Earth Angel-Supergirl was captured and imprisoned in the Garden of Eden by Lilith, the “mother of demons”. This left Linda-Supergirl with the problem of convincing everyone who had seen Supergirl “die” that she was, indeed, Supergirl. She acquired a new costume which she keeps to this day, but she now looks physically like Linda Danvers (shorter, smaller features, blond wig, etc).
The Quest for Supergirl (SUPERGIRL #51-74)
Linda continued to kick butt as Supergirl from issues #51-74, travelling with Buzz (former demon turned sidekick) and Mary Marvel and following the Chaos stream in search of the missing Supergirl/Earth Angel. Finally in issue #74, Linda confronted Lilith in a big showdown, including a heated battle between Linda-Supergirl and Earth Angel-Supergirl (who was being controlled by Lilith). The Angelic Supergirl broke from Lilith’s hold and destroyed Carnivore once and for all, Buzz killed Lilith, and Linda died briefly. Matrix/Supergirl in her earth angel form offered to merge with Linda once again to save her, but Linda heroically told her to instead save the life of Twilight, who had been forced to work for Lilith and had at the last moment sacrificed herself to help defeat Lilith. A wise move, as Twilight has the power to bring the dead back to life, and when she merged with Matrix she became the new Angel of Fire and healed Linda, restoring as much of Linda’s “own powers” as she could (i.e. her Supergirl powers as seen at the beginning of the series, before the angelic powers appeared). So Linda is Supergirl in her own right, the duty of Earth-born Angel has been passed on to Twilight along with the wings of fire and remaining essence of Matrix. It appears that the spirit and power of Supergirl is wide enough to exist in both the superpowered person of Linda Danvers, and as a merged entity with Twilight. Confusing is an understatement!
“Many Happy Returns” – The Kara Arc (SUPERGIRL #75-80)
Following the end of the “Quest for Supergirl” saga, yet another Supergirl appeared in issue #75. This is the original Kara from Krypton, and she caused quite a bit of confusion with her claims that she is the true Supergirl and cousin to Superman. Since Kara died in the Crisis on Infinite Earths and all memory of her was wiped from the remaining inhabitants of Earth (including Superman’s), her appearance was a great mystery to the post-Crisis Supergirl (Linda Danvers). Kara was indeed who she claimed – Supergirl, Kal-El’s cousin – detoured into the post-Crisis universe on her way to the pre-Crisis Earth. This bit of retroactive rewriting of past comics posits that the Supergirl we know from the Silver Age (known in DC continuity as “Earth-1″; Golden Age characters were said to inhabit “Earth-2″) was diverted into our current continuity. The Silver Age world Kara was destined for is definitely not the one we know, as Linda Danvers discovers when she lives there for a while. It’s like living in an idealized version of the 60′s! No one swears, the villains are always easy to defeat, and everything’s very, very clean. A utopia for comic book characters, one in which Linda Danvers has a perfectly happy life in which she marries Superman (she refuses to be anyone’s “secret weapon”!) and has a daughter named Ariella. At the end, however, she must return to her rightful universe, and Kara must return to her predestined place in history in a Silver Age world, with no memory of her time spent on Linda’s modern Earth.
The “Many Happy Returns” arc was intended by the writer, Peter David, to kick-start a new direction for the comic which would feature the three Supergirls: Kara the original Supergirl, Linda Danvers taking on the mantle of Superwoman, and Power Girl. (Revealed by David in If Supergirl Had Continued). Who wouldn’t have loved that? However the series was prematurely cancelled and David was forced to wrap up the story in issue 80, giving us a bittersweet ending to Linda’s and Kara’s story. Linda Danvers gives up the mantle of Supergirl and leaves Leesburg for parts unknown. In a farewell letter written to Clark, she bequeaths him her Supergirl mementos – and her beloved skateboard – and hints that she might adopt a new identity.
Following Supergirl Peter David moved onto a much darker series called Fallen Angel. The series stars a mysterious dark avenger named “Lee”, who is unusually strong and supernaturally-abled. After much reader speculation that Lee was the disillusioned Linda Danvers living on in that little-known corner of the comic book universe, Peter David revealed that Lee was not Danvers. However he introduced a new character named “Lin”, Lee’s predecessor, who is Linda Danvers in all but name (the series is now published by IDW, not DC Comics). See Wikipedia for further details on Fallen Angel’s Supergirl connection.