Beatrice Flowerchild began life as a human childhood companion to Julian Ainsworth, meeting him in elementary school. She vehemently rejected her birth name, choosing "Beatrice Flowerchild" instead. Bullied for her red hair, she gained confidence after Julian defended her, fostering deep affection for him. Her life ended abruptly when Fourth Holy Grail War debris crushed her moments before confessing to Julian.

The Ainsworth family resurrected her consciousness within a doll body, mirroring other mages in their service. Assigned the Berserker Class Card for the Fifth Holy Grail War, she hunted Shirou Emiya after his ritual infiltration. Shirou wounded her with a Broken Phantasm, but she survived via the Berserker Card’s God Hand and retreated.

Her doll form sports unkempt scarlet hair tied at the ends, yellow eyes, and a fang-like mouth, typically clad in goth lolita attire with an umbrella. Partial Berserker Install enlarges one arm with bandages and a gauntlet. Full Install swaps her outfit for Norse-inspired clothing—fur cloak, boots, and a red skirt with an ivory buckle. As a human, she wore glasses and braided hair in a school uniform, later adopting plainer clothes to evade notice.

Her personality exhibits volatility: joy in violence, irritation when foes survive. She cherishes stuffed toys and Julian, though her affection for him can abruptly shift to violence, like decapitating toys stolen from Erika Ainsworth. Internally, she wrestles with her doll existence, masking vulnerability with pragmatic remarks about obligations to Julian. She mourns their irreversible changes and lost past dynamic. Crucially, her resurrection erased most memories and her capacity to love Julian, though she gradually reclaims these feelings.

During interdimensional clashes, she assists Angelica in retrieving Miyu post-Gilgamesh’s spatial rift, paralyzing enemies with Mjölnir’s lightning. She later ambushes Illya and Tanaka in Ainsworth castle using her Berserker Card. Defending Pandora’s Box in Fuyuki, she battles Berserker-Installed Illya and Bazett, dominating until Illya invokes Thor’s poisoning myth. Beatrice remains unaffected—her card channels Thor’s son Magni, whose divine core rejects retreat. As she prepares a lethal Noble Phantasm, Bazett pierces her heart with Fragarach under Rin’s command.

The blow triggers Magni’s lightning rampage, nearly killing her. Shattered Julian memories surface mid-chaos. Illya intervenes, embracing Beatrice and urging her to hold onto feelings for Julian. Using her own Berserker Card’s failsafe, Illya force-ejects both cards, halting destruction. Defeated, Beatrice fears becoming an emotionless doll or monster. Illya confronts her again, prompting Beatrice to admit she lost her love for Julian upon resurrection and sees herself as a "disposable pawn." Illya counters that Beatrice unconsciously reignited that love, causing Beatrice to break down in realization.

Post-timeline reset resolving Darius Ainsworth’s global mud crisis, Bazett returns Magni’s card. Beatrice initially refuses, explaining each Mjölnir activation erases a memory. Bazett insists sacrificing the past enables progress, noting Beatrice retained her reborn love despite repeated card use. Beatrice accepts, and her card shifts from Berserker to Rider. Summoning Gullfaxi, she combats mud constructs alongside Bazett, memory loss blurring her recollection of Illya. Reuniting with Miyu and Julian, she rescues Julian and urges Miyu to rejoin allies after fully recalling Illya and Chloe.