Dagger is a young man and a prominent first-string member of the Noah's Ark Circus, where he performs as the troupe's knife thrower, a role from which his stage name is derived. His background is rooted in hardship; he was originally a thief living in the alleys of London's East End, abandoned due to having a missing right leg since birth. Along with other similarly "defective" children, he was eventually taken in by Baron Kelvin, and it was within this group that he and the others decided to form their circus family.

Dagger possesses a flamboyant, dramatic, and energetic personality, almost constantly maintaining a cheerful demeanor and a smile. He is amicable and prefers lighthearted atmospheres, often cracking jokes to dispel tension and assuming that anything he does not understand must be a joke. This friendliness is seen when he encourages newcomers to socialize, believing strongly in the spirit of teamwork. However, this amiable nature is secondary to his most defining trait: a deep, obsessive love for his fellow performer, Beast, whom he always calls "Sis" or "Big Sis." He is fiercely devoted to her, frequently serenading her, offering gifts, and becoming visibly jealous if anyone, such as Sebastian Michaelis, gets too close to her. His fixation is a constant, present even when Beast is not around, despite her never reciprocating his romantic advances.

In the story, Dagger, along with his circus family, is at the center of an investigation into missing children. Initially welcoming to Ciel Phantomhive and Sebastian as they attempt to join the circus, Dagger acts as an informal guide and teacher, notably attempting to instruct Ciel in knife-throwing and being astounded by the young earl’s natural talent. He is portrayed as a loyal and integral part of the group, following the orders of the ringleader, Joker, without question. His key relationships are central to his identity; his unrequited affection for Beast defines his personal motivations, while his bond with Joker and the other first-string members, including Jumbo, provides him with the only family he has ever known.

Dagger’s development culminates in a tragic act of self-sacrifice. During the assault on the Phantomhive manor, he is paired with Beast. After being pursued into the manor’s kitchen by Baldroy, who reveals a hidden machine gun, Dagger does not hesitate. He uses his own body as a shield to protect Beast from the hail of gunfire. Even as he dies from his wounds, he is shown crying but still smiling as he urges Beast to flee back to Joker, confessing that he had always wanted to take her "over the hill." This final act cements his role not just as a lovesick performer, but as a protector who gave his life for the person who mattered most to him.

In terms of abilities, Dagger is a highly skilled knife thrower who is said to "never miss his mark," a talent that is his main contribution to the circus’s performances. He is capable of using his daggers as effective weapons in combat, as demonstrated during the fight at the Phantomhive manor. Physically, he is noted for his distinctive appearance: he is slim, has brown eyes, and bleached blond hair with a frontal section left black. He wears stage makeup consisting of thick eyeliner with a painted dot under each eye. He is also missing his right leg, using an artificial limb created by the circus's Doctor.