Dagger is a young, slim male with sea-green eyes and bleached blond hair—the frontal section of it, however, is black. He has makeup around his eyes that consists of thick eyeliner, purple eyeshadow, and a painted red dot under each eye.
Dagger is typically dressed in his circus stage outfit, which consists of a long-sleeved, white and black striped shirt with a jabot, worn under a plain vest. He also wears black pants, over which is a black skirt, and a red and black hat. He is missing his right leg and wears the artificial one made by Doctor instead. In the past, Dagger had a band-aid across his nose for an unknown reason.
Dagger is a flamboyant, dramatic, and energetic individual who is almost constantly smiling. He seems to have a great love, bordering on obsession, for Beast, whom he always refers to as "Sis" or "Big Sis." He is always willing to express his feelings for her, frequently serenading her or offering her gifts. When Sebastian Michaelis has ended up too close to her by touching her leg, he was very upset, because he had not touched her yet. He continues these acts, even when Beast is not around and despite no indication of interest from her.
Dagger is amicable and demonstrates goodwill. He very much enjoys the company of others, as shown when he encourages Ciel Phantomhive and Sebastian to make new friends, too. When he does not understand what people are saying, he tends to assume they are joking. Seldom serious, he does not favor somber atmospheres and is quick to make jokes to disperse the tension. Even while dying, he is shown smiling.
Dagger 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 Dagger's case, he has been missing his right 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. Dagger, Joker, Beast, 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 Dagger 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. At his manor, a sulking Dagger sat beside Beast on a bench, observing as the other children play. Seeing this, Joker called Jumbo over. Jumbo invited Dagger to climb onto his back, and then he carried Dagger to the other children so that he could play with them. After living at Baron Kelvin's manor for some time, they decided to start a circus, and Joker gave him the stage name "Dagger," which initially he was not excited about. His real name is unknown.
After the show in the first-aid tent, Dagger has his prosthetic leg examined by Doctor. When Sebastian investigates Beast's prosthetic leg a little too closely, he assists her in attacking him, only to be later stopped by Joker.
The next day, Sebastian returns with Ciel to take the entrance exam. Dagger is shocked by Ciel's success at knife-throwing, and later at tightrope walking. Joker leaves them in the practice tent to stretch, and Dagger approaches them, admonishing them for being so sluggish. When questioned by a fellow member of the circus, Dagger informs that for today's performance he was the "top batter" and now he is just observing practice. He then asks Ciel and Sebastian what sort of show or "program" they would like to put on. He decides to take over teaching Ciel how to throw knives. When Sebastian shows that he is exceptionally skilled at every task, Dagger introduces them to Suit, someone who is equally skilled, who turns out to be William T. Spears. William then comments derisively on demons, and Dagger is initially speechless; however, he decides that William must be a joker with an interest for the occult. He encourages William and Sebastian to bond over their hopes for a great circus life; when William disagrees and promptly begins to leave, Dagger stresses that the circus is about teamwork.
Afterward, Dagger urges Ciel to begin practicing, while mistakenly thinking that Sebastian and William are getting along. Later, Ciel interrupts Sebastian and William's conversation, saying "that noisy knife thrower is calling." They return to the tent, where Joker and Dagger share the room assignments. When Sebastian and William are announced as being tent-mates, Dagger states that it is a great opportunity for Ciel to make new friends (he is paired up with Freckles) as he is already close to Sebastian.
Subsequently, Dagger has gathered with the first-string members, less Joker, after learning that Sebastian and Ciel had invaded their tents and left. Dagger suggests waiting for Joker to return from a visit to Father, but it is decided that it will take too long. They then agree to carry out their orders the following evening.
The next day, Snake approaches them and asks where they are going, and Dagger says he might be going on a date with Beast and that they will return the following morning. Snake says that his snake Emily will miss him and to hurry back, which seems to repulse Dagger.
That evening, the first-string circus members, less Joker, Doll and Snake, arrive at Phantomhive Manor, and he splits off with Beast to accomplish their task more quickly. Inside the manor, they hear sounds of a fight and assume that there must be a bodyguard. Near the staircase, they encounter Baldroy, and Dagger throws daggers at him. Baldroy blocks the daggers with a skillet and gets Finnian to aid in the attack. Dagger and Beast flee the area, narrowly avoiding the statues thrown by Finnian and bullets shot by an unseen Mey-Rin. Because of the danger, Beast suggests retreating. They attempt to navigate their way out of the manor and come into the kitchen instead. Baldroy reveals a hidden machine gun, and Dagger uses his body as a shield to protect Beast. His death is not instantaneous; in his last breaths, he, while crying, tells Beast to run to Joker and says that he had always wanted to take her over the hill.
- (To Sebastian Michaelis, about Beast) "You barmy bastard, how dare ye lay a hand on me sis's pale, silken skin! How dare ye. How dare ye! And when I haven't even laid a hand on it yet!"
- (To Beast) "I had... wanted to... take... Big Sis... over... the hill..."
Out of Universe
- According to the official Kuroshitsuji character popularity poll, Dagger is the thirty-first most popular character in the series, with 16 votes. He shares this spot with Alois Trancy, who also received 16 votes.