She is Julian's childhood friend, knowing him since elementary school. Beatrice Flowerchild is only a nickname that she picked for herself in the past when she was still alive due to the fact that she hates her real name. Her real name is unknown.
After being rescued by Julian when she was being bullied by boys of their school because of her red hair, she fell in love with him and gained more self-confidence. At a certain point in time she died in front of him, by being trapped beneath scrap and debris.
In the process, just like Angelica's, Beatrice's personality was placed into a doll body by the Ainsworth family, to serve as their servant. Being brought back to life alongside a few deceased mages, she is given the Berserker Class Card of the Fifth Ainsworth Holy Grail War in order to participate in the War and defeat Shirou Emiya, who slipped into the ritual.
Beatrice has the appearance of a young girl with rather unkempt scarlet hair tied at both ends, yellow eyes, and a fang-like mouth. She normally wears a goth lolita outfit while carrying an umbrella.
When she Includes with Class Card Berserker, one side of her arms becomes large covered in bandages and a gauntlet. When she Installs with Berserker, she wears a Norse theme outfit that consists of a fur cloak and boots, and a red skirt held by a buckle that resembles two ivory.
When she was alive, she wore a normal school uniform and glasses while wearing her hair in braids until she meet Julian.
Beatrice is shown to be highly volatile to the point of potentially bipolar. Appearing to take great joy in violence, she tends to get irritated easily if people survive her onslaughts. She has a love of stuffed toys and Julian Ainsworth, but her girlish affection for Julian can quickly turn into a violent depression in which she rips the heads off of stuffed toys she steals from Erika.
In the past, she never liked her red hair since she was always being bullied because of it, trying to make it plain looking to keep a low profile. As a doll, She is in conflict with her very own self. Instead of revealing her true thoughts on her actual situation and on Julian, she uses remarks that she doesn't wan't to say like commenting on the fact that she is obligated to help him because he already helped her when she was supposed to be dead. She thinks that both he and herself changed and she claims that she wants to go back to the time where things weren't like they are now.
Fate/kaleid liner PRISMA☆ILLYA 2wei!
She appears for the first time at the end of the series alongside Angelica, following the fight with Gilgamesh. When a rip in the world was created by Ea, she uses Mjölnir's lightning strikes to paralyze their opponent and allow Angelica to install the Archer Class Card. They then proceed to recover Miyu before they are again teleported to their own world.
In Fate/kaleid liner PRISMA☆ILLYA 2wei Herz!, they don't involve themselves after the fight and only appear at the very end of the series. They will only kidnap Miyu at the start of the anime adaptation of Fate/kaleid liner PRISMA☆ILLYA 3rei!!.
Fate/kaleid liner PRISMA☆ILLYA 3rei!!
She encounters Illya and Tanaka in a playground and proceeds to attack them with her current Berserker Class Card. After beating up Tanaka and noticing that she isn't normal, she decides to hit them with an ability from the card but is interrupted by someone who talked to her through unknown methods and then retreats.
After the two girls and Gilgamesh infiltrate the Ainsworth castle, she ambushes Illya and Tanaka in a room filled with mountains of Mystic Codes and other vessels, while Gilgamesh was facing Angelica. Clashing with Illya, she decides to install her card and use Mjölnir as she saw Illya was in the possession of the Caster Class Card, interfering in Gilgamesh and Angelica's battle. They chase the intruders up outside and then return to the castle under the order of Darius Ainsworth.
During the second assault of they opponent on their lands, she only gets in the battle after Julian Ainsworth's appearance as he was stabbed by Rule Breaker when he was acting as Darius's conceptual replacement, breaking the spell. She supports him as she destroyed Shirou's projected Ig-Alima, ejected him away from Pandora's Box and Julian and tried to fight Gilgamesh. She stays with Julian and Erika Ainsworth after their retreat.
As the Pandora's Box appeared an another time in Fuyuki City, Julian her the role to guard to pathway to the Box alongside Sakura. She faces Illya Berserker Install and Bazett alone on the first entry of the stairs, but gains the upper hand, overwhelming her opponents. Because of that, Illya uses the Berserker Class Inheritance to install Medusa Gorgon with the Rider Class Card.
Having exchanged blows, Illya tries to beat Beatrice by making her walk the nine steps backwards that killed Thor after being poisoned by Jörmungandr in order to reenact the legend. After making her do that, she notices that it didn't had any effect on her, as her divine core was telling her that retreating was unforgivable and to go forward because that was what his dad told him. It's revealed that the Heroic Spirit in the Card wasn't Thor but one of his sons, Magni. Losing more and more her sanity, she prepares to fire her Noble Phantasm in order to kill Illya. Rin knowing that Beatrice couldn't be saved, ordered Bazett to shoot her down while she was preparing to fire one last Fragarach. Her Noble Phantasm was affected by it and as she thinks about dying a second time for Julian, Beatrice's heart gets pierced by the sword.
However, instead of instantly dying, Beatrice starts to lose control of herself and release surges of lighting. Bazette deduces how the attack failed because the Divine Core of Thor, which Magni possessed to wield Mjölnir, acted as a conceptual defense and protected Beatrice at the cost of its own destruction. This leads to Beatrice losing the right to wield Mjölnir which causes the hammer to split and unleash Ragnarok. Throughout her rampage, her memories of Julian started flooding in. When the lost of control and Ragnarok's lighting were about to claim Beatrice's life, she begins to think back when Julian first saved her from bullies and the time they spent together before her death. To her sudden shock, Illya comes rushing to her rescue and embraces her declaring that Beatrice musn't let her feeliings for Julian fade. Illya ends up saving Beatrice by using the fail-safe that was installed in her own Berserker Card, force ejecting both cards out of both of them.
As Beatrice lies in defeat, she questions if she will return to a doll or lose her memories and become a monster. Beatrice then ask why Ilya was crying despite being the winner and realizes that Illya now knows her past. More of Beatrice past is then revealed on how she died by falling debris right before she was about to confess to Julian. Although her bottom half was crushed by the rubble, Beatrice barely regains her consciouness and sees Julian trying to rescue her. She tells him it's alright and to give up as she no longer can feel her body, but Julian rejects her pleas and still tries to save her. Right before her death, she hears Julian in pain and sees what appears to be Darius above him.
Her primary weapon is the Berserker Class Card holding the spirit of Magni, son of Thor. The card's Include mode is initiated by merging with her right arm morphing it into the massive arm of Thor, easily twice her size, and grants her his great strength.
Install mode grants Beatrice Magni's attire along with his arm and Mjölnir. Install mode allows her to fully realize Mjölnir's power as lightning strikes capable of breaking even atoms, however the hammer's power is limited if she channels it indoors.
She is also revealed to possess both Magni's Divine Core and Thor's, which Magni inherited after Ragnarok. Having Thor's Divine Core made Beatrice able to survive Fragarach, being killed but reviving on the spot from a lethal attack, but she lost the ability to wield Mjölnir since it was Thor's Core that allowed her to wield it.