Aelita Schaeffer (voiced by Sharon Mann) is the youngest girl in the group and the most naive of the five, having had comparatively less experience with the real world than her friends. She is thought to be an A.I. within Lyoko for the first two seasons. She is the daughter of Franz and Anthea Hopper, who designed both the supercomputer and Xana. She manages to settle into her new life by the start of the third season, making more jokes and generally displaying less naivety, and more reflected in her combat skills. Aelita is also capable of running the supercomputer in the event that Jeremy cannot. In the show it displays that she has feelings for Jeremy, but also has some affections for Odd. On Earth, her full name is Aelita Stones. In "Wrong Exposure" it is revealed that Aelita's true last name is Schaeffer, but that her father had changed it to Hopper, her mother's maiden name, to make a fresh start. Aelita's mother, Anthea, disappeared when she was young. In "Distant Memory, it was mentioned her mother, Anthea, was kidnapped by the Men in Black when Aelita was a little girl and was never seen again. For the first and second seasons, Aelita is 12 years old and in 8th grade, having been skipped ahead; for the third and fourth seasons, she is 13 and in 9th grade.

Aelita's hair is pink. She wears two main outfits throughout the series. Her daytime outfit in the first season is a maroon skirt, yellow shirt and pink jacket. She wears a dark pink jumper dress over a light pink hooded long sleeved shirt and dark pink knee boots as a trademark outfit for the both seasons. But the non-trademark outfit, her outfit consists of a dark purple dress that comes to her knees and light pink Quarter length leggings underneath. Her boots are a deep red. Her night clothes consist of white socks (sometimes barefoot) and an over sized pink t-shirt with a red letter "A" on the front.

For the first three seasons, Aelita's Lyoko attire consists of a pink mini skirt and a matching top that go over trousers and a long shirt that are both off-white, and matching shoes. She also has elf-like ears, with a small dangle earring in the left one. Her eyes are green. In the fourth season, Aelita receives a new pink form-fitting suit on Lyoko, which has a pair of retractable energy wings that enable Aelita to fly.

Her main interest is music, specifically audio mixing. Her childhood memories show that Franz Hopper taught her to play the piano at an early age. In the fourth season, she wins a contest to star in one of the Subdigitals' concerts. She, like Jeremy, is also a capable programmer and an excellent student. Aelita has four appearances on Lyoko. In season 1, she was a Lyoko being and wasn't allowed to be devirtualized and doesn't have any powers except for her "synthetization." In season 2, Aelita is a human being, but doesn't have any weapons. In seasons 3 and 4, Aelita can conjure pink orbs from her hands called "Energy Fields," and can be devirtualized out of the scanners without Code Earth after her human memory is regained.

Aelita's main power on Lyoko is her "synthetization," which allows her to alter small portions of Lyoko's terrain at will. She can use the same power to create clones of herself, though they have limited functionality, acting in a pre-determined manner. As mentioned earlier, Aelita also possesses the power to deactivate the towers on Lyoko, the main component in stopping Xana's attacks in the real world. In the third season, Aelita is able to defend herself by generating pink orbs called "Energy Fields," which can destroy any monster on contact, regardless of where they hit, excluding the Megatanks. Two energy fields can also be touched together to form a makeshift shield. Additionally, Aelita can charge her "Energy Fields" into a much larger ball of energy, which can do more damage than a normal Energy Field attack. Over time she also developes pink wings, which she uses for transportation; she can use them at her own will. For the first two seasons, she could not be devirtualized normally as the others could, requiring a special program to bring her to Earth, but after that is able to leave in the same manner as the others.