Hakaru Shiratori serves as a supporting character and the twin brother of renowned singer Kotori Shiratori. He possesses pale grey hair, purple eyes, stands around 170 centimeters tall, and frequently wears glasses. His personality is upbeat and mischievous, marked by a readiness to tease others, including openly acknowledging his twin relationship with Kotori despite the unwanted attention it brings.

Born into a wealthy Kyoto family, Hakaru and Kotori shared an early passion for music, both aspiring to become musicians. This trajectory halted when he contracted tuberculosis, requiring a three-year recovery that left him physically and emotionally exhausted, leading him to permanently abandon music. This decision fractured his relationship with Kotori, as he felt unable to fulfill their shared dream. Though he claimed Kotori ceased smiling after this, Hakaru himself never genuinely smiled again until later events.

Hakaru enrolls in the same school as Tamahiko Shima and immediately befriends him, becoming Tamahiko's first genuine friend despite Tamahiko's social isolation due to disability and family background. He defends Tamahiko from bullying and visits his home, meeting Yuzuki Tachibana. During one visit, he overhears Tamahiko lying to Yuzuki about his school life but keeps the truth confidential, understanding Tamahiko's noble intentions. Later, Hakaru facilitates a reunion between Tamahiko and Kotori, bringing her to Tamahiko's villa so she can seek inspiration for a love song. Their stay results in a quarrel where Kotori reveals Hakaru's abandoned musical aspirations. Following Tamahiko's encouragement, Hakaru reconciles with Kotori by performing a duet, symbolically supporting her career while accepting he will not return to music.

Subsequently, Hakaru volunteers at the hospital of Tamasuke Manabe, Tamahiko's uncle. There, he interacts frequently with Tamahiko's younger sister, Tamako Shima, who is training to become a doctor. Their relationship starts contentiously due to his rapid adaptation to medical work and Tamasuke's praise of his potential, which irritates Tamako. However, Hakaru develops romantic feelings for her and humorously proposes marriage during her 15th birthday party after she voices doubts about her marriage prospects. Their relationship deepens through correspondence when Hakaru travels to Germany as a medical apprentice; he sends Tamako letters with kiss marks, playfully requesting reciprocation, confirming their romantic involvement.

While training in Germany, Hakaru and his mentor are summoned by Liselotte Meggendorfer to diagnose Tokoyo Kurosaki, who suffers from tuberculosis. He corrects Tokoyo's misconception that the disease is hereditary, explaining it is contagious and manageable through robust nourishment and rest. Alongside Tamako, he becomes one of Tokoyo's primary caregivers, overseeing her treatment, which includes sun exposure for its germicidal properties. When Jintaro Shima is drafted into the Second Sino-Japanese War, Hakaru and Tamako assume responsibility for Tokoyo's ongoing care.

After completing his apprenticeship, Hakaru returns to Japan, marries Tamako, and establishes their own clinic. They continue supervising Tokoyo's convalescence for several years until Jintaro's postwar return. Hakaru and Tamako later attend Jintaro and Tokoyo's wedding, cementing their ties to the extended group.

Hakaru's name (策) translates to "measure, policy, or plan," while his surname Shiratori (白鳥) means "swan," or separately "white" (白) and "bird" (鳥). He shares several traits with Tamahiko, including poor artistic skills and an interest in the same adult magazines. His character arc concludes with him achieving his medical career and sustaining a lasting marriage to Tamako, with whom he operates their clinic.