Bazett is a Mage's Association First-Combat Specialist: Sealing Designation Enforcer originally sent to retrieve the Class Cards by the Mage's Association. She easily defeated Archer and Lancer, obtaining their two Class Cards, but Zelretch intervened by relieving her of her duty and replacing her with Rin Tohsaka and Luviagelita Edelfelt. They were given the two Class Cards and all rights to the mission. Due to a power game in the Association, she later set out to return and claim the seven Class Cards from them.
Bazett is dedicated to her work, and has little concern for anything outside of that. To this end, she shows little sentiment toward opponents or friends, and has no qualms about allying with recent foes where it suits the objective. By nature of her specialty, she will literally come to blows with anyone obstructing her, even if they should be children.
Her unshakable work ethic is a point of strength as well as a source of comedy relief, as she nonchalantly accepts a string of grueling service jobs, from maid cafe waitress to beach vendor. Remarking at one point that she would roadside grass if it were necessary, Bazett is rarely phased by her fiscal plights, having accepted them as par for the course in her violent tendencies. When the main characters find her sleeping under a cardboard box on a bench, she merely cites Japan's abundant climate before considering lodging with anyone.
As an enforcer, she does not care that Ilya and Miyu are fighting, but quickly voices her disapproval at the fact that Tohsaka and Luvia are involving children that are seemingly unrelated to the struggle.
Bazett arrives in Fuyuki after the situation with Kuro has been settled, planning to retrieve the Class Cards. She first targets Luvia in her mansion, easily dispatching her and Auguste. She feels disappointed after having heard of the Kaleidosticks and not getting the chance to experience their power. Luvia attempts a final stand and Rin also joins the battle, but they are easily defeated without touching her, with the exception of Rin placing a seal made from Ilya's blood on her, and the mansion is left in a collapsed heap. Taking the four Class Cards that had been stored there, she makes her way to leave and encounters Ilya and Kuro.
She marks them as people involved, moving to dispatch Kuro in an instant. Kuro manages to retaliate, but Bazett destroys her weapons and throws Caladbolg II back at her, identifying it as a tactic already used by Archer. She recognizes Magical Ruby as a Kaleidostick, and she tells Ilya she will guarantee her safety should she stand down from fighting and give her the other three Class Cards. Ilya refuses, transforming and surprising Bazett due to the information possessed by the Association not showing the real state of confusion caused by the Holy Grail War.
The state of affairs matters little in accomplishing her task, so she attacks Ilya and completely overwhelms her. Ilya is left unable to retaliate until she places defensive barriers on Bazett's arms to temporarily bind her in order to launch a strike, but she is able to smash them in time to dodge. She does note that the strike could have defeated her if the bindings had be doubled, and before Ilya can strike again, she raises a chunk of earth to block Ilya's field of view and lands a decisive strike on her by smashing through it.
She collects the Lancer Card from the fallen Ilya, but Kuro rejoins the battle after having recovered. The previous fighting had thrown the Class Cards carried by Bazett onto the ground, so Ilya tries to collect them during the fight. Bazett easily defeats Kuro and collects the other cards in the time that it takes Ilya to grasp the Rider Card. Although Bazett threatens her with destroying her wrist, Ilya refuses to give the card to her. Before Bazett can follow through with her strike, she is attacked by Miyu who then Installs the Rider Card.
Bazett is initially overwhelmed by the power of Miyu and her summoned pegasus. After Miyu questions about the whereabouts of Luvia, she unleashes Rider's Mystic Eyes and trump card, Bellerophon. This works to Bezett's advantage, allowing her to utilize Fragarach to stab the pegasus through the heart and nullify the attack. She easily dispatches Miyu to end the Install and claim the Rider Card. She then collects Miyu's Saber Card and then approaches Kuro for the Archer Card. Ilya also steps into the fight with a plan of her own.
Bazett strikes the ground to throw up debris to cloud the area. Kuro attempts to strike her with multiple thrown Kanshou and Bakuya and a strike from behind, but she easily destroys all of them and strikes her down. That allowed Ilya enough to time get in close to Include the Assassin Card held in Bazett's pocket, turning Ruby into Ilya and giving her enough time to get behind Bazett and reach for the seal left by Rin. Ilya is unable to make it, but interference from Luvia causes Bazett to dodge enough for her to reach. Properly forming, the same seal that was placed on Kuro, Rin explains the situation to Bazett.
Bazett calls her bluff, knowing that she could still easily continue to fight as long as she avoids killing. Rin instead reveals the existence of the eighth Class Card, causing Bazett to call a truce in order to gather more information from the Association because the situation has advanced beyond her what personal judgement can decide. She returns three of the cards in exchange for being able to learn of the card's location, and she leaves. Luvia's influence cause her to be personally billed for the damage, leaving her credit maxed out and her travel expenses being her only available funds.
She takes up a job selling ice pops, and later coincidentally meets Ilya and the others at the beach. She intimidates them into buying overly-expensive ice pops, and later tries to sell them more during the confusion caused by Ilya's group fighting over Ilya's favor in relation to Shirou. After the situation grows even more chaotic, she is hit with a tranquilizer from Ruby and later made to forget the preceding events. She was unable to sell all of her wares, and due to being unable to explain herself, her pay is docked.
She meets Rin before the battle with the eighth card, and they discuss the possible involvement Ilya and Miyu may have with the situation. Upon preparing to go to the Mirror World, she reveals that the curse was lifted by Caren, noting that it was not particularly strong in the first place. She plans to work independently from their group, turning it into a contest to claim the card. Ilya's group fails to damage Gilgamesh, so Bazett makes her move. She is quickly overwhelmed by the Gate of Babylon and her inability to permanently damage him, but her Rune of Resurrection, tenacity, and help from Kuro and allow her to build up power and strike his heart.
Though she manages to remove the second Archer Class Card, it is immediately sucked back into him, and the entire group is forced to retreat from the Mirror World by Ea. Bazett declares it impossible to win against him, and decides to plan a different strategy. She continues to travel with the group after Gilgamesh escapes into the real world, noting that it is seeking some form of Holy Grail unrelated to that of the Einzberns. She is only able to watch the battle between Ilya and Gilgamesh as she stands by with Kuro and the others. Once the battle ends, tearing a hole between the two worlds, she is left unable to act and brought over to the parallel world.
Bazett is extremely physically strong, casually tossing furniture with a single hand, causing great collateral damage from the force of her physical strikes, and easily uprooting large sections of the ground. She can match Heroic Spirits with her bare hands due to her honed fighting skills, requiring little effort in taking out most of her enemies. She can destroy projected Noble Phantasms with ease, even easily catching Caladbolg II and throwing it back, and deflect magecraft casually with her fists.
Her gloves are imbued with runes to form a Rune of Armoring that gives toughness greater than tungsten steel, and she can use them to enhance her strength, allowing her to smash through defensive magecraft that is too strong for her regular strength. Her body is one capable of withstanding any attack, and she can be called a "berserker woman." When faced with nine major stab wounds, including two fatal wounds and one directly to the heart, she has a Noble Phantasm-class Rune of Resurrection capable of instantly restoring her body upon her heart stopping.
Her trump card is Fragarach, a Noble Phantasm that works by erasing the trump card of the opponent, such as Noble Phantasms, after they utilize it by going back in time and then striking their heart before it is activated. It works against Miyu's pegasus instead of her when she utilizes Bellerophon. It is nullified by Rin's Death Agony Slavery curse, which causes any damage, including death, to the Master, Ilya, to be replicated onto the servant, Bazett. If Bazett would die at the same time as the enemy, it would create a causality conflict to say "Bazett fires Fragarach before she is killed by Fragarach." The curse does little to stop her from fighting, as she can simply beat Ilya within an inch of her life while ignoring the pain to herself. It is also easily removed by skilled magus such as Caren.