Supergirl

Kara In-Ze, aka Supergirl, is a character from Justice League and is a major character in The Knights of the Multiverse, introduced in Season 2. The last Argoan and Superman's foster cousin, she joins the Crystal Knights in their quest to protect the Multiverse, both to stop the ongoing crusade of Axis Zeon and in feeling that she had never really fit in on their Earth.

Personality
Most believe that since they both wear the "S", Supergirl is just as much of a girl scout as Superman is a boy scout. This is most certainly not the case. In reality, Kara is a very playful spirit, often acting very juvenile and impetuous, which doesn't offer much affection from her more serious compatriots. She also has a bit of a temper, always eager to fight thanks to the training she recieved from Argo's military in her youth with a no-talk, no-nonsense mentality in battle, so much so that often she'll put neutralizing a threat above saving others. Thanks to her lack of time on Earth, she also doesn't have the same blind love and dedication to humanity that Clark does. She does understand that life is worth preserving, but at times she doubts if humans deserve to be saved at all.

It's not she doesn't care though; she's (usually) well-intentioned and obviously loves her adoptive family a lot, especially Clark. She's just always had a hard time adapting to her new surroundings and finding people she can really connect with. At her core, Kara's homesick; she doesn't feel at home on her Earth and like she doesn't belong. Unlike Clark, who's lived as a human his whole life with no memories of his original home, Kara still remembers her's; all the friends and family that were left behind. So with her memories of home still fairly fresh in her head, as one can imagine, she has a much harder time relating to humans than her cousin did, often finding herself isolated. A part of her wants to believe she can find her place on Earth like Clark did, and while her has found a few companions among the League, she obviously doesn't have the same notion of time as Clark did, so she's often consumed by grief and it's very hard for her to even attempt to find comfort on Earth. She holds a lot of pain and rage inside of her that she constantly feels the need to repress and push does, which often comes out of her whenever she fights.

Pre-Series
Kara In-Ze originated from Argos, Krypton's sister planet. When Krypton exploded, Argos was jolted from its orbit into a much further orbit, causing the planet to freeze over. Kara was one of many that were stored in a hidden cryogenic facility and placed in suspended animation; unfortunately, over time, the facility would sustain extensive damage and everyone else in the facility froze to death, including her mother.

When Superman found her, he brought her back to Earth with him and introduced her to his adoptive parents, where they took her in for three years under the guise of Jonathan Kent's teenaged niece. Eventually though, she was inspired by her cousin's heroics as the Man of Steel, and made a costume similar to his, taking up the name "Supergirl" and occasionally visit Clark in Metropolis and join him on missions, even going up against Darkseid's Furies and teaming up with the heroes in Gotham.

A few years later, she was one of many heroes that were recruited into the new Justice League, though her somewhat immature nature made her act impetuously, which was frowned upon by some members of the League, especially John Stewart, though she did find a few friends among their ranks like Green Arrow. She would assist the League in many of their battles against dark forces, including their scuffle with Cadmus.

Skills & Powers
Because Argoans and Kryptonians sprang from a common humanoid ancestor millions of years before and both lived on planets orbiting a red sun, Supergirl's powers closely parallel Superman's, including superhuman strength and speed, near-invulnerability, super hearing, flight, super breath, heat vision, as well as x-ray vision, telescopic and microscopic vision.

Because Supergirl was much younger than Superman, her power isn't yet equal to her cousin's. By her 21st birthday, it was evidenced how much her power has grown, as her strength seemed to be near his. Additionally, Supergirl shared Superman's vulnerability to Kryptonite, was weakened by red sun radiation, and presumably has the same problems with magic.

Clark
Though she doesn't appreciate how often she feels like he's babying her, Kara cares about Clark a lot, the two having a solid sibling-like bond.

Dani Phantom
Kara found herself a kindred spirit in young Danielle, both often being seen casted in the shadows of their worlds' more famous heroes and feeling like they can't live up to their legacies.