Willow Rosenberg - Buffy the Vampire Slayer
Of all the long term characters of Buffy the Vampire Slayer, Willow went through the most changes during the series. Early on, she dated men, and relied mostly on her technical skills to help Buffy and the rest of the crew fight vampires. By the end, she had discovered more about her sexuality and began a deep, romantic relationship with Tara Maclay. Through this, she was also able to tap into her ultimate power as a witch, which led to some rather spectacular results late in Season Six. Willow is completely awesome in every way!