Birk is the son of Borka, the chieftain of the rival Borka robber clan, and his wife Undis. He is a young boy, approximately ten years old, as he was born on the same night as Ronja, the daughter of the rival chieftain Mattis. His family, the Borkas, have moved into the northern side of Mattis’s fort, which they rename Borka’s Keep, intensifying the long-standing feud between the two robber bands.

Birk has a distinctive appearance, with shoulder-length orange or red hair that falls in bulky strands and striking blue eyes. He typically wears a blue tunic and has a dark brown belt wrapped twice around his waist, which holds a pouch and a sheath for his knife. He is sometimes seen barefoot but also wears shoes on occasion.

In terms of personality, Birk is a cheeky, fun-loving, and highly rational boy. He serves as a contrast to Ronja’s more fanciful and impulsive nature, as he tends to be realistic and level-headed, able to calmly assess any situation he encounters. He possesses a gentle charisma that can be frustrating to those who wish to dislike him, and he often meets insults with a serene smile or a cocky smirk. For most of the story, he shows little emotional variation, with anger being the emotion he expresses most passionately. However, during his time living apart from the clans in the wilderness, some of this serene demeanor fades, and he begins to show feelings of grief and jealousy.

Birk’s primary motivation is survival and, later, personal freedom. Initially, he is simply a boy living within the constraints of his family’s bitter rivalry with the Mattis clan. As the story progresses, his friendship with Ronja becomes the central driver of his actions. He is resourceful and takes initiative to survive the harsh winter when his side of the fort is starving. Together with Ronja, he ultimately rejects the cycle of violence and feuding, deciding to leave both clans to live independently in the forest.

Birk’s most significant role in the story is as Ronja’s counterpart and eventual best friend. He is the child of the enemy, and their relationship forces a reevaluation of the senseless hatred between their families. He is also a catalyst for change, as his capture by Mattis leads to a hostage exchange where Ronja offers herself to the Borkas, an act that results in her being disowned by her father.

The key relationship in Birk’s life is with Ronja. They begin as enemies who mock and challenge each other, but after Ronja saves his life, they form a secret bond. This friendship deepens when Ronja secretly provides him with food during a harsh winter, saving him from starvation. Their loyalty to one another leads them to run away and live together in a cave, where they rely solely on each other for companionship and survival. His relationship with his father, Borka, is strained, as Borka is a cruel and unforgiving rival to Mattis, though he does care deeply for his son’s welfare.

Throughout the series, Birk undergoes significant development. He evolves from a smug, rational boy who accepts the feud into which he was born, into a compassionate and independent individual who rejects his family’s way of life. Living in the wilderness with Ronja, he learns to appreciate the beauty of the natural world and becomes more open with his emotions, laughing, expressing fear, and showing a pacifist ethic. His final act of development is choosing a peaceful life outside the robber clans, breaking the cycle of violence.

Birk’s notable abilities include his physical agility and fearlessness, demonstrated when he competes with Ronja by jumping across the dangerous chasm known as Hell's Gap. He is also a skilled user of his knife, which is his most important tool for fending for himself in the forest. Furthermore, he displays considerable resilience and survival skills, managing to survive a winter of near-starvation before taming wild horses and establishing a home in a cave with Ronja.