|First appearance||Kadic Bombshell|
|English voice actor(s)||Jodi Forrest|
Brynja Heringsdötir is Sissi's pen-pal from Iceland. She was only seen 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 arrives at Kadic, all the boys in school are attracted to her and all the girls wanted to know her beauty secrets. This makes Sissi jealous because she is no longer the "most popular girl in school". At first, Odd was not very interested, 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 start dating on the same day they meet. That afternoon, Odd introduces Brynja to the group. Like every other boy in the school, Ulrich and Jeremie are entranced by her good looks.
Aelita and Yumi are 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 group finds out they get angry and banish Odd from 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." Implying that her father's name is Hering.