Kazumi Akiyama is the younger sister of Wataru Akiyama, a skilled racer from Saitama known as the "Levin driver". She is a young woman with brown eyes and black hair cut to shoulder length, appearing to be a teenager. Her first impression can be somewhat fiery; when she initially meets Itsuki Takeuchi, she misinterprets his interest in her brother's car as mockery and becomes angry with him. Despite this rough start, she is generally described as being sweet and good-natured, which ultimately draws Itsuki to her.

Kazumi leaves her home in Saitama to work at an inn in Gunma Prefecture that is run by a relative. While working there, she crosses paths again with Itsuki, and the two begin to spend time together and grow closer. She acts as a connector between the racing scenes of Gunma and Saitama, helping to arrange a meeting between her brother Wataru and the talented downhill specialist Takumi Fujiwara. Having grown up watching her brother race, she possesses a considerable amount of knowledge about cars and racing, often picking up information while riding alongside him. This understanding allows her to follow the technical aspects of the battles she witnesses.

During her time in Gunma, Kazumi experiences a difficult situation at the inn where she is working, leading her to leave her position. This event, coupled with her family's concern, prompts her to eventually return to Saitama, though not before developing a meaningful connection with Itsuki. Later, she reappears in the story when the racing team Project D travels to Saitama for a series of battles. It is during this time that the complexities of her personal life come to light, as she becomes torn between her growing feelings for Itsuki and the return of an older ex-boyfriend who wants to reconcile. This situation ultimately leads to Itsuki being heartbroken, showing that Kazumi is not entirely naive and has a complicated romantic history. Her relationship with her brother, Wataru, appears to be close and stable, though she is not afraid to stand up to him when she believes he is being unreasonable or overly obsessed with his hobby.