Daiki Asuka, known as Asuka Junior, excels academically at St. Paulia's Academy with sharp intellect but struggles socially and fails to grasp romance or interpersonal dynamics, especially with girls. His physical strength and motor skills lag behind his analytical mind. As the son of Detective Tomoki Asuka, he accesses police connections. Mocked by classmates for failing to capture the phantom thief Saint Tail, he develops an inferiority complex and dedicates himself to apprehending her to prove his worth.

Initially dismissive of Saint Tail's altruistic motives, he later acknowledges her honorable intentions after firsthand observation. He secures her promise to exclusively send him advance notices of her capers via calling cards and never allow another to capture her. This agreement fuels his obsession with uncovering her identity, especially after noting behavioral parallels between Saint Tail and his classmate Meimi Haneoka. His suspicions grow when observing Meimi's reactions during heists, leading him to project his developing romantic feelings for Meimi onto Saint Tail.

His investigation turns personal as he shifts from wanting to arrest Saint Tail to protecting her from external threats like rival investigator Rina Takamiya, who uses aggressive tactics including tear gas. His protectiveness peaks in a high-stakes bet with Takamiya: if Meimi is Saint Tail, he dates Takamiya; if not, Takamiya ceases harassing Meimi. Saint Tail's successful deception of Takamiya resolves the conflict. Emotionally torn, Asuka Junior grapples with Saint Tail being treated as an abstract ideal rather than a person, deepening his concern for Meimi's well-being. He recognizes his pursuit is tied to shielding Meimi, culminating in a love confession and the start of their relationship.

Meimi's dual identity is exposed when antagonist Rosemary reveals her as Saint Tail and kidnaps Asuka Junior. During four days of captivity, he reflects on Meimi's secret support, interpreting her calling cards as expressions of her feelings. In the confrontation, he physically rescues Meimi during her "final theft" as Saint Tail, metaphorically "catching" her by absolving her vigilante duties, leading to Saint Tail's public disappearance.

Eight years later, Asuka Junior becomes a private detective continuing Saint Tail's mission to protect Seika City using tips from Seira Mimori’s confessionals, mirroring Meimi's methods. He marries Meimi after proposing, keeping her past as Saint Tail secret. Physically, he undergoes a growth spurt, becoming notably taller than Meimi, and adopts a professional demeanor fitting his career.