- Kara In-Ze was born and raised on Argo, where she thought she might follow in her mother's footsteps and become a doctor someday.
- When she was sixteen, the planet froze to death around her, thanks to the event that shattered Krypton, and she was put into stasis to wait things out.
- Eventually Superman found her in the only intact stasis pod, and brought her to Earth.
- Kara lived in Smallville for three years, with periodic visits to Metropolis (and occasionally Gotham) and exploits as Supergirl; she donned the costume out of admiration for Clark's activities and a deep-seated need to use her newfound strengths for good. "I watched one world die around me, and I was powerless -- I won't stand by again when I have the power to help people in need!"
- Kara met Barbara Gordon, and the two of them fought some crime together and forged a friendship that has lasted many years.
- As Supergirl, she fought a variety of villains, most notably Darkseid (who beat the bejeezus out of her, twice) and a brainwashed Clark (same result). Despite all this she was determined to see her chosen path through.
- After Clark put her on life support, she was cared for by Professor Emil Hamilton, who saved her life -- and took samples of her DNA for cloning.
- Eventually Clark deemed her adjusted enough to Earth life, skilled enough with her powers, and responsible enough to be trusted with Justice League membership. She was immediately one of the more powerful members of the League in terms of sheer brute force, but had a great deal to learn.
- But Kara learned, both from the school of hard knocks and a great deal of intensive training from Green Lantern and Green Arrow, who were both her mentors and friends, once she earned their respect. League training and missions became her life, and she grew steadily more distant from any sort of civilian identity, while becoming more and more dedicated to her work.
- During her first year in the League, Supergirl started having terrible nightmares in which she killed people; this was eventually revealed, with help from Green Arrow and the Question, to be a psychic connection to Galatea, a Cadmus clone whom the government had trained to be a living weapon. Tea was better trained with a fully mature body, and utterly determined to kill the original to prove her worth as an individual.
- When the founding members of the League turned themselves in to US custody after the Watchtower fired on Earth, Kara held down the fort in space. Ultimately she confronted her copy and defeated her, as usual, with brains as much as brawn.
- Time passed, Kara grew, and became increasingly active worldwide, achieving a wide following. She changed her uniform, which had been homemade and extremely simple, to be more like her cousin's, whom she was extremely proud of. By her 21st birthday, Jon Stewart admited there was little more he could teach her; she had power, control, tactics, and excellent judgement.
- Kara needed all of these traits to survive a trip to the 30th century, where she single-handedly held off the entire Legion of Super-Heroes (minus Brainiac 5, plus Jon Stewart) long enough for her allies to deactivate the brainwashing controlling them. Having started to feel more and more like an Outsider on Earth, Kara was surprised by how much she felt at home in the future.
- Brainiac 5's extreme cuteness didn't hurt matters; it was love, or at least crush, at first sight. She decided to stay in the 30th century, where she felt she could make more of a difference than in the hero-deluged 21st.
- However, not long thereafter, the future was drastically and apocalyptically altered overnight by some sort of time manipulation. Kara, being from the past, was the natural choice to send back to try to set things right.
- Kara is deeply worried about these events, but also aware that it could well be years before the time villain reveals himself; she's doing her best to find herself, and her place, back on Earth, while patiently searching for signs of the trouble.
Kara is fundamentally an outsider on Earth in a way that Clark is not, and having to deal with that informs a great deal of her personality. It is incredibly frustrating to live in a (relatively) low-technology time; and unlike Clark, Kara remembers, directly, the catastrophic events that killed her entire planet and everyone she knew.
In general she uses those memories to drive her to fight tirelessly to defend those who cannot defend themselves. She buries her trauma under a playful spirit, a bright, enthusiastic and occasionally cocky personality. She is hip and independent, and endlessly eager to explore the Earth she now calls home.
Kara would die before allowing any harm to come to her adopted world, and has nearly done so, many times, stepping up alone against powers orders of magnitude out of her league because there's no one else who can. "Suicidally brave" is not an unreasonable description of her dedication to protecting those she cares about -- which is everyone. Her deep-seated sense of responsibility defines her as little else does; she is, at the beginning and end of the day, goodhearted.
She has a reputation for being headstrong, and there's something to this, though Kara has learned how to take orders gracefully over the years and has become a great deal more mature. Her straightforward self-confidence sometimes rubs people the wrong way. She no longer looks before she leaps quite as drastically, having been seasoned by experience. She is even occasionally suspicious of others' motives, slower to trust than some heroes are, in part because she's been betrayed several times by those close to her.