Beast is a young woman with red eyes and black hair, styled in tight curls. She possesses a curvaceous figure; her most distinctive feature is her endowed chest.
Beast wears a revealing, black leather bodice, and a short, black leather skirt with a red sash. She also wears thigh-high fishnets, held up by a garter belt, which stop at diamond-adorned garters. She has matching fishnet gloves that extend to the area below her shoulders. Her various accessories include a red headband, and her makeup consists of dark red lipstick, fake eyelashes, and little red marks or stones painted under her left eye.
As part of her circus outfit, Beast nearly always carries a whip. In addition, because she is missing her left leg, she has an artificial limb in its place. In the past, during her time at Renbourn Workhouse, she sported a simple dress and her hair was worn in waves.
Beast is a very domineering, aggressive, and ill-tempered individual; when irritated, she is quick to lash out and resort to violence. Furthermore, while she has a close bond with the other first-string members, Beast particularly loves and cares deeply for Joker, even though she is fully aware that he will never return her affection.
Unlike the other first-string members, Beast does not seem to hold Kelvin in the same high regard. She is also more willing to abandon the other children back at the Renbourn Workhouse, as indicated when she asked Joker if they can run away from Kelvin.
Beast was born on May 22, 1864, without a left leg, as Mally to a scullery maid named Amelia. She grew up in an alley in the East End, referred to as a gutter, with the other first-string circus members, less Snake. Joker stated that they came together because they were each defective in a way. In Beast's case, she has been missing her left leg since birth.
Ten years ago, Doll rushed up to Joker one day with a tooth in her hand. She gleefully exclaimed that it fell out and promised she would buy them all bread later, believing that she would later get a coin from the Tooth Fairy. However, she was oblivious to the fact that it did not actually exist. Beast commented that if Doll did not get her coin next morning, she would be extremely upset, to which Joker decided that he should attempt to earn something. Beast, Joker, Dagger, Peter, Wendy, and Jumbo then proceeded to work on manual chores earnestly all night long in order to earn a single coin for Doll. They were relieved to see they made it on time as they placed it beside her whilst she was sleeping. The next morning, an oblivious Doll showed off her coin to them, stating how happy she was that they could eat bread with it and an exhausted but happy Beast smiled along with her. However, in the belief that the Tooth Fairy would give her more coins, Doll later attempted to take out more teeth that weren't wobbly, to the dismay of the others.
As thieves, they were unsuccessful, and they barely managed to survive. At some point, Baron Kelvin found them in the alley and adopted them. While there, Beast seemed to have developed her crush on Joker, as she was touched by his offer to carry her on his back if he still had both arms, and was very glad to be in his presence.
At a circus show that Ciel Phantomhive and Sebastian Michaelis attend, Joker introduces Beast, the animal tamer, as the main act of the show. Sebastian is called down on stage as a volunteer. However, he ignores Joker's orders to lie down, and instead directly interacts with the tiger, Betty. When Betty bit him, Beast uses her whip to try and get the tiger to release him. However, Sebastian stops her from hitting the tiger and states that Betty did nothing wrong. He admonishes her that if she simply recklessly swing the whip, she will never be able to train Betty, much to Beast's chagrin.
After the show, Beast spots Sebastian backstage in the first-aid tent, where she reprimands him for ruining her performance. However, Doctor tells her that she is supposed to be the professional, and if she could not control Betty, then that was her fault, not Sebastian's. Beast concedes, albeit unhappily, and then requests that Doctor look at her prosthetic leg. Sebastian finds the artificial limbs intriguing and grabs Beast's leg to investigate. However, he ends up looking a little too closely for her comfort, and she angrily calls him a pervert. Dagger and Beast begin to attack him, but he is able to skillfully evade all of their attacks. This impresses Joker, who stops Beast and compliments her leg to calm her down. Joker invites Sebastian to join the circus.
After Ciel and Sebastian have both infiltrated the circus, the circus is set to put on a show again. Ciel uses this time to investigate the first-string members' tents, while Sebastian is called to stand in for Wendy in the show. While watching Sebastian's act, Beast's top breaks, forcing her to return to her tent to get a new one. Ciel was in her tent at the time, but Sebastian's speediness allows him to finish his act and beat Beast back to the tent, in time to hide both Ciel and himself.
When Joker learned of their presence in their tents, he leaves to go see Father. Beast catches him while he is leaving, and asks him to escape from Father with her, assuring that they will be fine since they have the circus. He dismisses her, and she hugs him from behind. He gives her his scarf, tells her to go to bed, and momentarily departs.
Sebastian reveals that he was spying on them, and asks if she is crying, which she is. Although she attempts to push him away, ultimately, he is able to inveigle her with his words. They move to Beast's tent; Sebastian seduces her while asking for information, and she discloses to him her feelings regarding Joker and Father. Sebastian urges for a name, which she eventually gives.
The next morning, Sebastian and Ciel leave the circus, and that night, the remaining first-string members, less Snake, hold a meeting. They decide to carry out Father's orders, even without Joker. Beast offers to go tell Joker herself, but Wendy insists that she is needed for the plan. Doll is sent instead, and while Beast helps Doll get on a horse, Doll apologizes for putting the circus at risk. However, Beast blames herself instead, which confuses Doll. Later while packing, she asks Wendy if she is positive that they cannot wait for Joker, and Wendy tries to comfort her, telling her that she can talk to Joker soon.
The following evening, the first-string members arrive at Phantomhive Manor and split up. Beast goes with Dagger into the main house, where they hear sounds of the battles Wendy, Peter, and Jumbo have. When they encountered Baldroy, Finnian, and Mey-Rin, they decide that the bodyguards are too powerful, and make an effort to retreat. Instead, they end up in the manor's kitchen, where Baldroy is waiting for them. He exposes his hidden machine gun, and Dagger saves Beast by using his body as a shield. Finnian comes to rescue Baldroy, who reveals that all of the flour in the air is now his weapon. He lights it with a match, and she dies in the subsequent explosion; in her last moments, she thinks of Joker. Later, her scarf is thrown away by Sebastian, as he cleans the manor.
- (To Joker) "Let's give this a rest. We've got this circus. I'm sure we can get by. So let's wash our hands of this business and run away to where Father can't find us."
- (To Sebastian Michaelis, about Joker) "It's been this way for as long as I can remember. He just never says the words I long to hear."
Out of Universe
- According to the official Kuroshitsuji character popularity poll, Beast is the thirty-eighth most popular character in the series, with 7 votes. She shares this spot with Queen Victoria and Paula; both also received 7 votes.