As a child, Sakuya Watanuki experienced a traumatic family tragedy when his older sister took her own life. Their parents, seeking financial gain, covered up the event as an accident to collect her life insurance payout. Although Sakuya was aware of the truth, he was pressured into lying to the authorities, marking the first of many lies he would be forced to tell. This event left him with deep guilt, as he came to believe he bore responsibility for his sister’s death.

Following this period, Sakuya encountered Tsubaki, the Servamp of Melancholy. Perceiving the weight of Sakuya’s guilt, Tsubaki assured him that his sister’s death was not his fault. Acknowledging the boy’s suffering, Tsubaki promised that he would one day “come pick him up once everything is over.”

During his adolescence, Sakuya’s life ended in the same tragic manner as his sister’s. After his death, Tsubaki fulfilled his promise by turning Sakuya into a vampire and making him one of his subclass.

Although his exact age is unknown, Sakuya Watanuki has the physical appearance of a 15-year-old boy. He has green hair and is frequently shown wearing green-themed clothing, which complements his distinctive look. After becoming a vampire, his eyes turned red, marking his transformation into one of Tsubaki’s subclass. A notable feature of his design is a pair of pink headphones, which he is often seen wearing and which serve as a recognizable part of his character’s image.

Among his classmates, Sakuya Watanuki is regarded as the class clown, often making jokes and spreading rumors about local vampire sightings to frighten others. His playful and mischievous behavior masks the darker aspects of his past and his hidden identity as a vampire. The only person who truly knows who he is beneath this facade is his best friend, Mahiru Shirota.

Sakuya’s character is defined by conflicting loyalties. While he values his friendship with Mahiru, he remains deeply devoted to Tsubaki, believing that Tsubaki would never betray him or any member of his subclass. This conflict leads to moments of emotional strain, particularly when Sakuya learns that Mahiru has lied to him. In response, Sakuya pretends to despise Mahiru, hoping to provoke him into killing him, underscoring his complex mix of loyalty, guilt, and despair.

As a member of Tsubaki’s subclass, Sakuya Watanuki possesses abilities that distinguish him from both ordinary humans and typical vampires. Unlike most subclass, he is able to walk in sunlight without suffering harm, a trait unique to those created by Tsubaki.

Through the use of magic, Sakuya can manipulate parts of his clothing as extensions of his body. His suspenders, for example, can be elongated and used to restrain opponents or to summon weapons, such as the chainsaw he created in one confrontation. This versatility makes his combat style both unpredictable and adaptive.

Sakuya’s signature ability is “I Will Die Yesterday by Your Side,” also referred to as The Five Second Hypothesis. This technique allows him to generate physical illusions of himself that act as distractions. These duplicates can speak, bleed, and behave as he would in a given situation, making them indistinguishable from the real Sakuya until they are destroyed, at which point they dissolve into petals. While the ability is active, Sakuya can reposition himself out of sight, reappearing once the illusion vanishes.

The name The Five Second Hypothesis references a belief that decisions and judgments are often made within a narrow, instinctive window of time. The ability exploits this by forcing opponents to accept the illusion as reality until it is disproven, creating a tactical advantage in battle.

Tsubaki
Sakuya met Tsubaki, the Servamp of Melancholy, sometime after his sister’s death. Aware of the guilt Sakuya carried for lying to the authorities and believing himself responsible for his sister’s death, Tsubaki reassured him that the tragedy was not his fault. He promised Sakuya that he would “come pick him up once everything is over.” Six years later, when Sakuya died in adolescence, Tsubaki fulfilled this promise by turning him into a vampire and one of his subclass.

Even under Tsubaki’s service, Sakuya continued to struggle with the trauma of his past and the cycle of lies surrounding his family. Despite his conflicted emotions, he remained loyal to Tsubaki, convinced that Tsubaki would never betray him or any of his subclass. This loyalty often placed Sakuya in conflict with his bond to Mahiru Shirota.

Mahiru Shirota
Sakuya is introduced as Mahiru Shirota’s best friend, frequently spending time with him and two other classmates after school. However, it is later revealed that Sakuya manipulated Mahiru’s memories, making him believe they had been childhood friends when, in reality, they had only known each other for a year. Despite this revelation, Mahiru continued to view Sakuya as a genuine friend.

As Mahiru learned more about Sakuya’s past and inner struggles, his determination to oppose Tsubaki grew, vowing to free Sakuya from the promise binding him. While Sakuya initially befriended Mahiru out of boredom, their bond deepened, and he eventually valued Mahiru’s friendship more than his obligations to Tsubaki. Their relationship strengthened further when Mahiru reached out to Sakuya with understanding, prompting Sakuya to feel gratitude for being “found” by someone who truly cared for him.