Her name is Taiyo, and she, along with her brother Yami, are the dragons of light and darkness that give power to Lyoko's core. However, Yami was corrupted (making the evil we know today as XANA), and Taiyo was forced into the real world. To watch over Lyoko, she was the creator of the super computer, along with Franz Hopper.
Ever since the computer was shut down, Taiyo had taken residence into Lyoko's core until someone reactivated the super computer.
Once it was reactivated, Taiyo flung herself back into the real world, and enrolled herself at Kadic. She became a good friend of the main characters, and William obviously. She developed a secret crush on him (^^), and hung out with him every day in their free time.
Once she felt it was time, she told the main 6 (not counting William, he didn't know yet) about her TRUE identity, and continued to fight XANA with them. She had not told them that the same evil was her brother, though.
Once William was possessed by XANA, Taiyo was the one who tried the most to release him. Eventually, they fight Yami himself, and he sends William. Shooing the others away, Taiyo insists on fighting him herself. Once a little while had passed, Taiyo could feel a small bit of the true William being released. Eventually, William's mind was torn between good and evil. And, this is when it happens. Since the OC is for him, I'll let you guess.
Once he is released, with the power of everyone's strength, Taiyo reforms herself as the Dragon of Light. Yami becomes the Dragon of Darkness, and the two begin to battle. Once Yami is defeated and free, everyone prepares for the moment-shutting down the super computer.
But, there is a twist. Both Yami and Taiyo must re-enter Lyoko's core at the same time the supercomputer is shut down. William insists on bringing the siblings to the core. Once there, Taiyo has her final "certain moment" with William, then she and Yami jump to the core. William is devirtualized, and the two siblings are sealed away.