Supergirl

Supergirl is also known as Kara Zor-El, cousin to Superman. No familial connection with Superboy.

Background
Kara Zor-El was born on Krypton approximately 15 years before it's explosion. Her father, Zor-El, was brother to Jor-El, Superman's father, making the two cousins. On Krypton she did not have a cousin to take care of and is the lone child of her parents whom treated her well in the Kryptonian fashion. She was raised on science, math, art, reason, and a pragmatic philosophy that kept their society going for so long but in the end prevented enough people from listening.

Before the planet was destroyed, Kara was sent off in a spaceship to find earth and be there before her cousin arrived. Instead, due to errors in the trajectory and computer, she was instead sent off course and eventually found herself on Earth decades after her cousin had landed. He was raised as an earthling, and she was raised as a Krypton, but early on they both met and grew close. The only two survivors of the horrible things that had happened to their planet, or so they had thought.

Superman had found Kara and chased her down on the first few days, the yellow sun's radiation worked quickly, even more so because her space pod had radiation chambers in them to develop her superhuman powers even before she arrived on the planet. The anti-superhero act was still in place at this point in time, and Kara was sent off to the Amazonians to learn to control her powers and fight. Wonderwoman I's sacred island had stood the test of time and was out of governmental controlled space, and so she was free to use her powers and learn how to fight in martial arts.

Superman, during this time, was working very hard to get the act removed, and all the while Kara was growing up. She donned the symbol of her family and helped her cousin when she could, though bits and pieces of her past kept creeping out into the open and made quite the splash when they did. On more than one occasion an overwhelming urge to kill her cousin came about, trying to make her fight him, distorting her past somewhat telling her that she was sent to kill him. In the end she never got that bit solved, but Superman did succeed in getting the act removed and then disappeared. Kara hasn't had many nightmares or personality shifts since his departure.

In past few years Kara has grown up, from the 15 year old girl she was when she arrived to an eighteen year old girl who's been learning to control her powers. She has stepped into a few aliases in that time, tried out an alter ego of being the daughter of Lois's sister, and giving up on all of the alternate selves she tried to invent. Like any teenager she's finally feeling a bit of her own self coming through, pulling herself up and away from the other superheroes and trying to live a normal life as an actual alien from another planet instead of pretending to be a normal human. This means most of her friends are superbeings, and because of that there is the other unique issue of constantly getting into trouble.

Superboy and her have been the closest, for obvious reasons, they share some of the same history of being tied to the symbol that Superman made. She doesn't like the status that has been thrust upon Superboy making him more involved with big government and being that large icon of hope. They were best of friends always hanging and doing normal stuff because they couldn't do super stuff so easily, but he has more important things to do now and visits rarely. She's left alone again, there's crime to fight and she participates in the bigger battles to help prevent supercrimes but she tries to avoid getting involved as much as she can while she figures out her own life. Many teen groups have attempted to get her to join up with them in the short time since they've been legal again, but many people find all too soon that Supergirl's in a rut. No friends, no family, and constantly torn with teenage depression afflicting her at the worst moments. Hopefully this all turns around for her soon and she can meet some supers that will be her friend, and maybe she can get out of her rut.

Personality
Genuinely Good. Despite everything she's been through and all of the potentials of being sent to kill her own cousin, or whether or not that was just poison, her evil half being separated out at one point for her to know that such things existed within her, all of it... Kara Zor-El is still a good person. She wants to help others, save them, prevent harm and violence from affecting all the people she gets to know. She would rather save a villain than hurt them or have them imprisoned, so long as she could save them.

Pragmatic. Kryptonian society is much more pragmatic and rational than earth. If someone is evil and they do harm you punish them, sometimes by removing them entirely. There are all sorts of non-lethal but just as permanent ways to get rid of someone with the technology level Krypton had and they would. In this way, Supergirl often fights with a willingness to do more harm to villains and surrounding areas than Superman is capable of even letting himself go to. She is more practical in these matters, and it probably did not help that Wonderwoman I had been her primary teacher while Superman was off negotiating with Luthor. All good people, but some more willing to harm others to get the good result than Superman would be in the same situation.

Teenage emotions. As any teenagers would be, Supergirl is very concerned about what other people think of her, and she reads all the news stories that feature her that she possibly can. This has resulted in angst, depression, and even a bit of pride from time to time. Many reports talk about her as having been a second rate superman, who has destroyed as many buildings as she's saved, even worse sometimes. For a while there before the superhero act was revoked, she was even thought of as a villain and began to feel like one. Other superbeings and their disposition toward her have a large impact on whether or not Supergirl is in a good mood or a bad one, she wants to be accepted and she feels very very alone most of the time. Like any teenager she's always checking out other people, comparing herself to other women, and that alone could ruin her day and send her into fits of tears. As she grows up she'll become more stable and hopefully on the better end of the stick.