Ylva has fluffy, wavy hair that is kept tied up with bangs framing her face. She has an expressive face and is seen as a tough woman.
The oldest child and due to the age gap between her and Thorfinn, Ylva had to assume a lot of responsibility for the work on her family's farm due to her mother's frailty. As such she has a more pragmatic outlook than her father and raised the idea of acquiring a slave to help with the labor.
Ylva was born in Jomsborg in 987 just before Thors left to fight in the Battle of Hjörungavágr. She was a difficult birth for mother Helga. Nonetheless, Thors was disappointed that she was not a boy and was not very careful with his daughter at first. He tried to leave for Norway before naming her, but Helga was determined that he do so first--this was the only time Thors ever saw Helga angry. He opted for the first thing that came to mind, his mother's name. At this point, Thors' mindset concerning war changed drastically.
Three months after the Jomsvikings held a funeral for Thors after his apparent death in the battle, Thors returned to Jomsborg to take Helga and Ylva away with him. As they were leaving their house, they were confronted by Thorkell, who had been having a drink on the roof. Although Thorkell attempted to stop them from leaving and had to be knocked out by Thors, Ylva remained asleep during her family's escape.
When Thors dies and Thorfinn doesn't come home after escaping with his father on a warship, Ylva becomes the main supporter of the family, especially since her mother is of very frail nature. In fact, Ylva becomes known as quite a rebel and maniac girl by the village. This is probably because she has to fish, whale, and fulfill all the women's jobs too. Soon after the loss of two family members, she overcompensates by working. In the end, Helga has to teach her self-control. After later encountering Thorfinn in Gainsborough, Leif tried to convince him to return by revealing that Ylva had since married and now had a family.
Ylva is a multi-talented girl. She can fish, whale, knit, crotchet, and tend sheep.