Brynja Heringsdötir is Sissi's pen-pal from Iceland. Her first and only appearance in the series was in the episode Kadic Bombshell. She is voiced by Jodi Forrest with a thick Norse accent, but she is very good at speaking English and French. When she first arrived at Kadic, all the boys in school were attracted to her and all the girls wanted to know her beauty secrets. This made Sissi jealous because she was no longer "the most popular girl in school." At first, Odd was not very interested in her, but after getting a good look at her, he becomes obsessed with her. She also seemingly takes a liking to him as well, calling him the Icelandic word for "sweetheart." She and Odd started dating the same day they met. That afternoon, Odd introduced Brynja to the group. Like every other boy in the school, Ulrich and Jeremie are entranced by her good looks.
Aelita and Yumi were bitter and jealous that she is getting all the attention and act rather cold towards her. That night, the group confronts Odd about her to which he retorts that they are all just envious. Brynja overhears this and is slightly upset. Later that night, Brynja asks Odd if he could get nicer friends, however he denies this, saying that his friends are special - blurting out the truth about their connection to Lyoko. Brynja demands an explanation, so Odd takes her to the factory (blindfolded) and shows her the lab. In there, she couldn't contain her amazement and tries out the keyboard on the Supercomputer, not aware that it was, in fact, running a program. As soon as the others find out, they get angry and kick Odd out of the group. The next day, when Odd and Brynja are supposed to go to the pool together, Odd leaves to fight X.A.N.A. William, so Brynja breaks up with him, believing she was stood up. The next day, Sissi had Brynja sent to the countryside to stay with her cousin out of jealousy.
Under the system of Icelandic names, her last name means "Hering's daughter." This implies that her father's name is Hering.
- In the Polish version, she was voiced by Monika Wierzbicka.
|Jeremie Belpois | Aelita Schaeffer | Odd Della Robbia | Yumi Ishiyama | Ulrich Stern | William Dunbar|
|Sissi | Jim Morales | Suzanne Hertz | Anthea Schaeffer | Mr. Delmas | Laura Gauthier | Franz Hopper|
|Kiwi | Herb Pichon | Nicholas Poliakoff | Amelia "Milly" Solovieff | Tamiya Diop | Hiroki Ishiyama | Johnny | Takeo and Akiko Ishiyama|
|X.A.N.A. | Monsters|
|Professor Tyron | Ninjas | X.A.N.A. William|
|Jean-Baptiste Pujol | Theo Gauthier | Emily LeDuc | Taelia | Samantha Knight | Samantha Suarez | Heidi Klinger | Mathias Burrel | Magali de Vasseur | Claire Girard | Brynja Heringsdötir | Patrick Belpois | Romain Le Goff|
|X.A.N.A. Aelita | X.A.N.A. Odd | X.A.N.A. Yumi | X.A.N.A. Ulrich | X.A.N.A. Franz Hopper | X.A.N.A. Anthea Schaeffer | Jeremie Clone | Aelita Clone | William Clone | Subdigitals|