Marya 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
Marya grew up in Russia at the turn of the 20th century. Her mother was a servant to Empress Anna. She also knew Rasputin. The Empress made Marya and some other girls learn to dance, with a brutal training regime. She loved it in theory, but it was painful. Then someone brought her to Free Country, where she could dance the way she wanted, but felt too clumsy compared to the fairies. However, she loves to watch them dance.
In Free Country, there are many kids who have escaped the real world. There, they can life forever as children, away from the danger and corruption of adults. Their goal is to bring all the world's children to Free Country so they can be safe.
Her friend 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 does not love him back.
Marya knows that the kids of Free Country need to find Tim Hunter, because he can open the magic gates that would allow them to bring in more kids. She grabs some apples, chalk, and dancing shoes, then travels to Earth by creating a hopscotch board and singing a song, which is the way the Free Country kids teleport. She starts looking for Tim, not knowing that he has just been kidnapped to another dimension by Tamlin.
Marya goes into a jewelry shop and tries to get a bracelet fixed, but the man recognizes it as a famous piece and thinks she stole it. She got it from her mother, who got it from Empress Anna. The man tosses her out, but the Punk Waitress sees what happened and gets her jewelry back. Then she gives her a drink in her cafe and helps her find Tim through the phone book. He's not home, but she gets his address. On the way she she sees a dance studio and is envious.
She finds Tim's house, and Tim eventually shows up. She convinces him that Free Country is better than his life with his frustrating father.
However, when Tim shows up in Free Country due to Marya's magic hopscotch game, the other kids don't realize who he is, and he walks off. The kids' plan to bring all children to Free Country is ultimately sabotaged by a monk from the Dark Tower.
Marya, however, did not travel to Free Country with Tim, and stayed on Earth, held fast by her visions of children dancing.
The Books of Magic
She ends up going to the dance studio, Swan's School of Dance, and practices there all the time. She gets a job at a cafe (presumably the one where the Punk Waitress works). Molly also dances at the studio, so they become friends. Molly stops by on a date with Tim, and they all meet. Marya tells Molly that Tim is a magician. Freaked out, Tim accidentally freezes them in time, but manages to unstick them.
A unicorn shows up, and Daniel, who has been banished from Free Country and transformed into a powered dirty version of himself, and who is angry at Marya and, by association, Tim, smothers it with his soot powers.
They clean it off and Marya rides off on it, to keep it safe. Tim fights off Daniel.
Marya keeps the unicorn at the dance studio. Daniel finds Marya there. They argue, and he slaps her. The unicorn stabs him with its horn. He leaves, and she tries to find peace in dance. She keeps the unicorn as a friend/pet.
She remains friends with Molly. Later, Molly complains about the crazy life it has been with Tim. To Marya, it doesn't sound so bad. Molly gets annoyed with Marya's innocence. Circe hears them talking and comes up. She says she can solve their boy problems. She turns boys into animals until they can get rid of their toxicity.
Daniel shows up. 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. They give money to a homeless man, who they don't realize is Tim's dad.