Asuka Langley Sohryu is a character in the anime (and manga) Neon Genesis Evangelion. She is a brash, opinionated and sometimes cruel 14 year old girl of Japanese, American and German ancestry who has been selected to pilot an Evangelion unit, a role that she is very proud of.
Although she is a central character, and perhaps manages to steal the show, she was not an initial character, being introduced in the eighth episode, one-third of the way through the series. The series so far had been built around the characters of Rei Ayanami and Shinji Ikari, whose range of expression went from downcast to despondent. The vivacious and cheerful Asuka immediately made the dynamic of the children much more interesting. Her egotism, which often came out as contempt for Shinji and Rei, was first used for comedic effect, and the results are (in my opinion) rather funny.
Neon Genesis Evangelion being what it was (which is a depressing mind screw), Asuka's outgoing demeanor is slowly peeled away to reveal insecurity and tragedy. Asuka only feels good about herself when she is the absolute best in the world, an impossible goal, and one which finally breaks her down. In the movie, End of Evangelion, she is finally able to face her fear of failure, and finds a newfound meaning to life. Very briefly.
Her main character trait is her extroversion and self-confidence, which is in exact contrast to Shinji's introversion and self-doubt. Philosophically, Evangelion is about how people cope with loneliness and isolation. Asuka is a study in how it can be ignored and overcame---at least briefly---by the mindset of being purely outward focused. What is ironic is that Asuka is in many ways what she thinks she is: intelligent, competent and very, very brave, and it is only her ego that gets in the way of her success.