Daniel was created by writer John Ney Reiber and artist Peter Gross in Arcana Annual 1 (despite what it says to the right abiout "first appearance"; Arcana Annual 1 comes before Children's Crusade 2).
Major Story Arcs
Daniel grew up in Victorian London, where he was a street kid and chimney sweep who worked for Reverend Slaggingham.
Eventually Daniel, Marya, and several other children from the real world ended up traveling to the dimension of the Free Country, where they can life forever as children, away from the danger and corruption of adults.
Daniel likes to take trips to Earth to see what kinds of strange things they have now, although he wouldn't want to live there. He is in love with Marya, and brings her a statue of a dancer that he stole from Earth, because she wants to be a dancer. She appreciates the statue, but ends up leaving Free Country to go to Earth, where she can learn to dance.
The Books of Magic
Later, Daniel accidentally kills another kid after being teased for liking Marya, and is banished from Free Country. Back in London, he starts to look for Marya, but can't find her.
Angry at how London has changed, he goes in the sewers. There, he finds Reverend Slaggingham, now a cyborg trying to suck the happiness out of the surface world. They find a unicorn has invaded Slaggingham's underground space, and shoot at him. The unicorn runs off.
Slaggingham mentions Tim Hunter, and Daniel gets mad--he thinks Tim stole Marya away from him. Slaggingham agrees to help Daniel, and transforms him so that his soul is now on the outside--he looks like a dirty chimney sweep, and has powers.
The unicorn shows up where Marya is; she is with Tim and Molly. Daniel attacks the unicorn with his soot, choking and soiling it.
Molly kicks him in the groin, and they clean off the unicorn. Marya rides off on it, to keep it safe.
Daniel tries to burn Tim, but Tim absorbs his fire. Daniel is crushed at his failure. They show him a reflection of what he looks like now, and Daniel is further shattered. He asks Tim to fix him. Tim tries, but can't, because it's who Daniel really is now.
Tim makes Daniel take him to Slaggingham, whose cyborg body has fallen apart. The sewers are flooding, and Daniel leaves Tim to drown.
He finds Marya back at the dance studio she has been practicing at, with the unicorn. They argue, and he slaps her. The unicorn stabs him with its horn. He leaves.
Later, he finds Slaggingham's head and talks with it. He watches plays like Hamlet and avoids people, since he is still stuck in his dirty form. He senses the use of magic in the park, and wants to see if Marya is there.
He finds her, Molly, and Circe there. Circe gets rid of his dirtiness, but also turns him into a cute dog so he can't hurt Marya. Marya loves him as a dog and brings him home with her. Molly thinks this is a problem, because he'll still be needy about Marya, but now she'll accept it.
Much later, at the end of the series, we see Marya hanging out with the Punk Waitress, who has a small baby with her. Marya has Daniel the dog on a leash.
In one alternate reality created by Tim, several Books of Magic characters act as members of a Teen Titans-esque team called the Mystic Youth. Daniel goes by the name Danny the Running Boy, who is "able to run fast." They are all killed by Tim's alternate-reality self, The Other.
Powers and Abilities
Daniel used to be able to travel back and forth from Free Country using a hopscotch board and a song like all children, but then he was banished.
When Slaggingham transformed him, he gained powers connected to being a chimney sweep. He could send snaking trendrils of soot through the air, he could cause fires, and he could teleport in a flash of soot.