Birk Borkason is one of the central characters in the anime Ronja the Robber's Daughter. He is a young boy who is born on the same stormy night as Ronja, making him the same age as her. He is the only son of Borka, the chieftain of the rival Borka robber clan, and his wife Undis. Raised within his father's clan, Birk grows up in the forest, learning the ways of the robbers and living in the shadow of the long-standing feud between the Borkas and the Mattis clan.

In appearance, Birk has pale skin and striking blue eyes. His most notable feature is his shoulder-length red or orange hair, which falls in thick, bulky strands. His typical attire consists of a blue tunic and a belt that wraps around his waist twice, from which he hangs a pouch and a sheath that holds a knife. Unlike Ronja, who often goes barefoot, Birk is usually seen wearing shoes, though he will occasionally be barefoot as well.

Birk's personality is a blend of cheeky, fun-loving energy and a grounded, rational mindset. He tends to be realistic and pragmatic, often assessing situations calmly and thinking on his feet, which contrasts with Ronja's more impulsive and fanciful nature. He possesses a gentle charisma that makes it difficult for others to dislike him, even when they are determined to do so. For much of the series, Birk shows little emotional variation, often seen with an even, serene smile or a cocky smirk, with anger being the most passionate emotion he typically expresses. However, he is also described as being somewhat sensitive and thoughtful, with a slight tinge of cynicism in his expression. Despite his generally calm exterior, he can be stubborn, especially when it comes to spending time with Ronja.

Birk's primary motivation is his growing friendship with Ronja. Despite being raised as enemies, he is drawn to her cheerful and frank personality. He is loyal and caring, with a strong sense of wanting to do what is right, even if it means defying his own family. He demonstrates humility by being able to admit his faults, a trait not common among the proud robbers. He also shows a deep concern for the well-being of others, such as when he takes action to protect Ronja from danger. While he is the pride of his father Borka due to his cleverness, Birk himself seems unsure of his parents' regard for him and eventually decides that he does not wish to continue the life of a robber.

Birk's role in the story is that of Ronja's closest ally and best friend, eventually becoming like a brother to her. Initially, they meet as rivals, with Birk engaging Ronja in a dangerous game of jumping across the chasm that divides their families' halves of the fortress. After Ronja saves him from falling, their relationship begins to change. He later saves Ronja from the magical lure of the harpies, an event she initially forgets due to their enchantment, but this cements their bond. Their friendship deepens during a harsh winter when Ronja secretly brings him food, and they promise to always be there for each other. When Birk is captured by Ronja's father, Mattis, Ronja offers herself as a hostage to the Borkas in exchange for his freedom, an act that leads to both children being cast out by their respective fathers.

Key relationships define Birk's development. His relationship with Ronja is the most significant, transforming from enmity to a profound, platonic bond of sibling-like love and loyalty. His relationship with his father, Borka, is strained; although Borka cares deeply for his son, their interactions are often cool, and Birk feels a lack of emotional warmth, leading him to leave home. Similarly, his relationship with his mother Undis appears distant. Birk's time living apart from his clan in the Bear's Cave with Ronja is a turning point in his development. During this period, his usually serene demeanor melts away, allowing him to express a wider range of emotions, including grief, jealousy, genuine fear, and open laughter. He also begins to voice an appreciation for the beauty of the natural world around him, showing a more relaxed and vulnerable side.

Birk does not possess any supernatural or magical abilities. His notable skills are those born of necessity from growing up in a robber clan in a dangerous forest. He is practical, level-headed, and an excellent thinker in a crisis, able to calmly assess any situation he is faced with. He is capable of using a knife and a spear for both utility and protection. His survival skills and his rational, observant nature are his greatest assets as he navigates the perils of the forest and the conflict between their families. Ultimately, his journey leads him and Ronja to reject the violent, feuding ways of their parents and to decide that the life of a robber is not for them.