Haruka Tanizawa is a student from the third-year class at Shiroiwa Junior High School who becomes a participant in the Program. Before the competition begins, she is known as the tallest girl in her class, standing at 172 centimeters, with short hair and an athletic build. She plays as an attacker on the school's volleyball team, a role where her intelligence often compensates for any lack of raw strength. Her appearance and demeanor are sometimes described as boyish, a quality that becomes especially noticeable when she stands next to her petite friend, Chisato Matsui. Her best friend is the class representative, Yukie Utsumi, and she is also close with classmates such as Chisato Matsui, Satomi Noda, Yuka Nakagawa, and Noriko Nakagawa.

At the start of the Program, upon witnessing the violent death of a classmate, Haruka strongly advocates for the formation of an all-girl group. Alongside Yukie, she helps gather several female students to take refuge together in a lighthouse, hoping they can find a way to survive without turning on each other. She is depicted as a loyal friend who places great trust in her companions and does not easily suspect them of ill intent. This trust extends to Shuya Nanahara, the male protagonist. When the group discovers him badly injured, another girl, Yuko Sakaki, accuses him of being a cold-blooded killer and demands he be left to die. Despite the growing tension, Haruka refuses to abandon Shuya, believing he could be an ally and trusting in his character.

The situation inside the lighthouse deteriorates after Yuko attempts to poison Shuya, a plan that accidentally kills Yuka Nakagawa instead. This event sows paranoia among the friends, leading to a violent confrontation. During the ensuing shootout, Haruka witnesses Satomi Noda kill both Chisato and Yukie. Even after being shot in the side by a burst from an Uzi submachine gun, Haruka manages to raise herself and fire a fatal shot at Satomi, dying from her own wounds in the process.

The supplementary manga, Battle Royale: Angel's Border, explores Haruka's perspective in greater depth. The story reveals that she had recently come to realize she held romantic feelings for her best friend, Yukie. Aware of her own attraction, she had been contemplating how to confess her feelings or ask Yukie out, though the onset of the Program leaves these thoughts unresolved. This internal conflict adds a layer of personal tragedy to her actions, as her fierce desire to protect the group and her loyalty to Yukie are motivated by a love she never had the chance to express. Throughout the ordeal, she demonstrates notable physical durability and a strong will to fight, as evidenced by her final, desperate act of retaliation against her killer.