Julian Ainsworth heads the Ainsworth family and serves as the central antagonist in *Fate/kaleid liner Prisma☆Illya 3rei!!*. He is Zachary Ainsworth's son and Angelica Ainsworth's younger brother. Originally destined to be the vessel for their ancestor Darius Ainsworth, Angelica perished in the Fourth Ainsworth Holy Grail War, leading Julian to replace her as the vessel around age eleven. This process fused Darius's consciousness into Julian's body, inflicting significant psychological and physical stress, marked by pronounced eye bags. Julian initially concealed his identity under Darius's conceptual replacement, adopting his ancestor's appearance and mannerisms.

Confronting the world's impending death, Julian sought mystical artifacts and vessels to substitute for Noble Phantasms and the Holy Grail. After numerous failed experiments, he identified Miyu Edelfelt as a viable Holy Grail capable of saving the world. He captured Miyu and initiated the Fifth Holy Grail War using Class Cards, intending to sacrifice her to ascend humanity for the new world. Miyu's brother, Shirou Emiya, fought to free her, enabling her escape to another world. Shirou was subsequently captured as leverage to ensure Miyu's return. The plan paused until Gilgamesh's Enuma Elish opened a path between worlds, allowing Julian's allies Angelica and Beatrice Flowerchild to recapture Miyu.

Julian's personality exhibits contradictions and instability. Before fully embracing his role as Darius's vessel, he displayed traits of a typical irritable, socially awkward teenager, disdainful of pointless conversation. He maintained a casual friendship with Shirou, though he often snapped at him and concealed his emotions, even enduring physical discomfort like spilling hot tea to appear indifferent. Discovering Shirou had hidden Miyu shattered this, leading Julian to abandon his glasses—a symbol of their severed friendship—and adopt a colder, vengeful demeanor. As Darius's vessel, Julian adopted a bipolar persona, shifting between calm, doting behavior toward his foster sister Erika and manic outbursts when his "legend" was threatened. He insisted Erika view him as her father, brother, and sole ally, yet punished her harshly for disobedience, such as spanking her when she endangered his narrative by having Angelica kidnap Illya prematurely. He perceived the world as a staged performance, with individuals like Illya and Tanaka as predetermined "players." He despised Shirou for interfering, declaring him irrelevant to his legend.

Julian employed psychological manipulation and coercion. He regularly extracted Miyu's blood to inscribe tablets for his ritual, threatening Shirou's safety to ensure her compliance. When Miyu resisted or expressed despair, he intensified her suffering, projecting mental images of Illya's battles to crush her hope. He believed Miyu's suffering was necessary for humanity's salvation, framing himself as a "Champion of Justice" willing to sacrifice one for the greater good—a philosophy Shirou directly opposed. His downfall began when Illya used the Caster Card's Rule Breaker during a confrontation, dispelling Darius's conceptual duplicate and exposing Julian's true form. This humiliation, compounded by Miyu's escape and Shirou's liberation, triggered a hysterical breakdown. Julian retaliated by summoning a cube-like object unleashing corrupted Heroic Spirits, attempting to lure Miyu back by threatening global despair. He clashed with Shirou in a final confrontation, where Shirou vowed to stop his plans.

Underlying Julian's actions was deep-seated hatred for Darius, whom he blamed for Erika's suffering. This resentment festered over years, driving his obsession with control and desperate need to validate his "authenticity" when labeled a "counterfeit." His declaration that his legend's conclusion was "already determined" reflected a pathological refusal to accept failure.