Code Lyoko: The New Girl (BK 1)

It's been a year since the Code Lyoko gang discovered the Supercomputer and the world of Lyoko, and they're still battling it out against X.A.N.A., the evil computer virus. Tired of always failing to connect the pieces to everything, and above all, struggling to survive the latest X.A.N.A. attacks while also juggling school, family, relationships, and making plans for life after Kadic, the group is surprised to find an unlikely ally hidden away in a seemingly invisible Lyokian tower.
This surprise goes by the name of Saphire Stonez and she claims to be Aelita's long-lost sister. She and Aelita were separated as kids when she was forced to live with their father, Franz Hopper, in the Hermitage before he sought refuge on Lyoko. Abandoned by her father in the sector that shouldn't exist, Saphire spent many human years living with a Lyokian family and amongst other Lyokian citizens from whom she had to hide her Earthian status. While she was allowed to have an education, create lasting friendships, and make a name for herself, once she discovered a human sister left behind on Earth, Saphire makes it her life mission to find her and protect her from all harms: Lyokian and Earthian.
While the group is initially skeptical of her explanation, when Saphire is able to prove she shares a connection with Aelita and Franz Hopper and reveals an extensive knowledge of the Lyokian world which is a lot grander and complex than the group originally found it, they have no choice but to (hesitantly) trust Saphire and let her into their group, their home, and Kadic Academy, hoping she'll be the key to defeatin X.A.N.A. While supporting them in the latest of X.A.N.A.'s attacks and restarting her life on Earth, Saphire proves herself to be a strong asset. But on the side, she's protecting dark secrets from her past and plotting new ideas for the future of Lyoko, and while she is doing everything in her power to keep her new family safe, her unhealed past might just be their undoing.