Lagertha

Only Chapter IX of Saxo Grammaticus' Gesta Danorum (commonly known as "History of the Danes") contains information about Lagertha (also known as Ladgerda). Ragnar Lothbrok, a legendary warrior, travels to Norway in order to exact revenge on the Swedish king for killing his grandfather Siward and humiliating his wives and relatives. In Saxo's words, "...among them was Ladgerda, a competent female warrior who, while a girl, had the courage of a man and battled in front among the bravest with her hair loose over the shoulders. They all offered to help him." Everyone was in awe of her peerless accomplishments since her flowing hair down her back revealed her gender.

She makes Ragnar so impressed that he chooses to make her his bride (he even directly credits her with the victory), but Lagertha has a bear and a hound posted outside her home as security against him. Ragnar kills the two creatures, marries her, and they have two kids together. However, later, when he realizes that she attempted to provoke the dog and bear into attacking him, he gets a divorce from her and marries Thora. About Lagertha, nothing else is known.