Mirja is a voluptuous young looking woman. Despite her youthful appearance, she is 42 years old. She has small horns above her head, pointy ears and a cow-like tail, like other Kýlbringa. She wears a dress that fully exposes her breasts with pasties covering her nipples. She also wears a bell.
Mirja has a motherly personality, even more pronounced that other Kýlbringa. She shows no shame in her attires and she's prone to ask embarrassing questions to Effie in order to find out Effie's feelings. She has a high sense of duty and responsibility.
When Rotte was a baby, she didn't like very much the breast milk of either her mother or the royal wet nurse. As the top wet nurse in Ygvarland, Mirja was called to become Rotte's wet mother. When Mirja breastfed Rotte, Rotte only took a little bit and appeared to dislike the milk. Hearing Olav's concerns about Rotte's health due to refusing to drink enough milk, Mirja assured him that she would do her best and would bring everyone in her village should it be needed, but, moments later, Effie, who Mirja had brought to learn the ropes as a wet nurse, suddenly starting producing milk and Rotte happily drank it. With that, Effie became Rotte's wet nurse.
Years later, when Rotte visits Effie's hometown with Asuha, Ini, Zelda, Naoya and Effie, Mrija welcomes them as the head of the chief of the village, surprising everyone with her bold attire. Inside her house, while chatting about why the small town is heavily protected and why the Kýlbringa are a national treasure, Mirja comments that there were two break-in attempts (by Bjarki, Kató and Níls, who were captured, and by Dóra and Sigurð, who managed to escape) a little before Rotte came. As Ini finishes explaining why Kýlbringa and their milk are highly appreciated, she comments that the milk she is drinking has a banana flavor, to which Mirja says that that's her milk. She also says that the Guð Mjólk of their village is famous for its hints of fruits and nuts. A little after that, Est arrives and Mirja introduces her. When Est sees Ini, she immediately takes Ini away to play with her. When Edda arrives short after, she takes Asuha, Rotte and Zelda with her to drink her milk directly from her breasts. While going away, she teases Naoya about going with them as well. Seeing Effie's reaction, Mirja immediately notices Effie's feelings and asks her if she has found herself a good man, which embarrasses Effie.
Effie, trying to deflect the conversation, ask her mother if she shouldn't be more worried about her older sister Edda. Surprising Effie and Naoya, Mirja answer that she is leaving Edda alone since at the moment since she seems to like girls more than men. Mirja expresses surprise at Effie not knowing this. Still trying to divert the conversation to another topic, Effie ask to talks about something else, so Naoya asks why Effie is called Elfie in the village, Effie comments that when Rotte was younger, Rotte couldn't pronounce "Elfie", so she called her "Effie" and that name stuck. At that point, Mirja recounts the story of how she was supposed to be Rotte's wet nurse originally.
After all the girls return, they all take a bath to rest. Mirja, bathing along with Effie, takes the chance to confirm that Effie indeed has found a potential Mr. Right in Naoya. Effie confirms that this is correct, but, since Rotte and Naoya haven't formalized their relationship, she is trying not to think about until they have children.