Jin Asukai is a university student with a quiet and contemplative nature, studying art at a national university while working part-time as a tutor and guidance counselor at a prep school. Despite his calm, clever, and careful demeanor, which makes him seem like a typical, approachable young man, he harbors a unique and profound ability. Since childhood, he has possessed the power to perceive the human heart in a visualized, symbolic form. He sees the hearts of others as plants, most often described as roses, and can instantly identify what an individual lacks in their life by examining the condition of this metaphysical flower. Within this vision, the petals represent brilliance, the leaves stand for kindness and gentleness, and the roots symbolize confidence.

For most of his life, this ability was a source of quiet frustration. He could identify the missing pieces in a person's soul but felt powerless to truly fix them, able only to offer advice or kind words in an attempt to compensate for what was lost. His perspective changes dramatically when he encounters a woman named Suiko Minahoshi, who calls herself the Imaginator. This entity, who exists as a formless will or a ghost-like presence, recognizes Jin's potential and offers him a way to transcend his role as a passive observer. The Imaginator persuades him to use his gift not just to see flaws, but to actively reshape human souls, eliminating pain and dysfunction to create a better world. Jin decides to act as an Imaginator in order to pursue this will, a decision that positions him as a central antagonist in the VS Imaginator story arc.

Motivated by a sincere but misguided desire to help humanity, Jin develops a grand plan to change the world. He believes that by forcibly rewriting the emotional makeup of people, he can remove the suffering that leads to ruin. His ambitions escalate to the point where he intends to use a synthetic human named Aya Orihata as a "seed" to spread this transformation across an entire city and, eventually, the globe. Jin's power allows him to interact directly with the roses of others; by touching or altering the petals, leaves, or roots, he can exert temporary control over a person's feelings and psyche. However, these manipulations are short-term effects and cannot permanently or completely transform a heart. His plan also involves the use of brainwashing serums and henchmen to achieve his goals.

Key relationships define his trajectory and eventual defeat. He works with the entity Spooky E, though his idealistic methods ultimately clash with other characters' brutal pragmatism. He develops a strategic alliance with Kyou and Nagi Kirima, using the nickname "the Proposer" to act as the thinker and strategist for the group. Later, he befriends Josuke Kikikawa. Jin also has a personal connection to his cousin, Kotoe Kinukawa, who harbors feelings for him; rejecting her is a significant action that helps break her free from other influences. His primary conflict is with the titular character, Boogiepop, who views his mission to impose salvation on humanity as a threat to the natural order.

Jin Asukai experiences significant development from a frustrated observer to an active agent of change. He is driven by a belief that his power must have a greater purpose and despairs at the world's inability to fix itself. Despite his calm and intelligent exterior, he is an emotionally vulnerable figure, which makes him susceptible to the Imaginator's manipulation. He is not a villain driven by malice, but a tragic figure whose compassion is twisted into ideology, leading him to conclude that violating free will is an acceptable price for a utopia. His plan ultimately fails when Boogiepop directly confronts and defeats him, revealing that the "perfect rose" he saw in Aya Orihata was a fake, as she is not entirely human. In the aftermath, it is suggested that he may eventually take a central role within the secretive Towa Organization.