Gemma Masters is the daughter of Cheryl and Tony Masters, and the niece of John Constantine. Both her parents, Cheryl and Tony Masters, are dead. She practiced magic, much to the dislike of her uncle, John Constantine. Alongside Chas Chandler, she is one of John's only loved ones to have survived throughout multiple periods of his life but whereas Chas is often thrown in at least one issue during every writer's run, Gemma is seldom used and the vast majority of her development happened while Mike Carey was at the helm of the Hellblazer ship.
Gemma was created by Jamie Delano and John Ridgway, first appearing in Hellblazer #4 (April 1988) wherein she was kidnapped by the Resurrection Crusade and rescued by her uncle John.
She's one of the few early Constantine characters to survive up until present day (most of Moore's were killed before Hellblazer even started and several writers would follow that example, killing the very cast members they created). Since Hellblazer has spanned over twenty years in real time, Gemma has aged from a ten year-old child (in her first appearance) to a matured adult, older even then John's wife Epiphany. The biggest period of her life so far explored was 2003-2005, which given John's real time aging should have been from the age 25 to 27.
Gemma is a practicing witch. Her father kills her mother when he is tricked by one of her uncle's demon children into thinking he is doing it for God (as he is an extremely religious man). He then tries to kill Gemma but her uncle John arrives and stops him. Then her uncle goes to Hell to retrieve her mother's soul. But he is unable to bring her back, and as her father committed suicide, Gemma is left an orphan.
She has recently got an invitation from Terry Greaves to her uncle's wedding to Epiphany Greaves. Gemma has given up the occult, and is unsure about attending her uncle's wedding, unsure if it will make her want to go back into that lifestyle, and talks to her therapist about it. Her therapist believes it could be a good step for her. She see's her uncle's ghost and is very rattled about this, but she does attend her uncle's wedding. Demon John (who had replaced John at the ceremony) corners Gemma in the bathroom (Gemma believing it is her uncle). After this Gemma swears to make John feel the same pain she is now feeling.
Gemma hires a witch to summon a demon to attack her uncle. In this time she also steals John's trench coat and sells it.
She lures John to an abandoned warehouse where the demon had been summoned. John arrives and Gemma leaves. Gemma goes and finds Epiphany, and she tells Epiphany what happened. Epiphany, knowing it was Demon John at the wedding, tells Epiphany that it wasn't John who raped her it was Demon John. They stop at Epiphany's lab to grab a concoction that Epiphany had made. While here Gemma notes that though Epiphany is younger than her Epiphany has her life together more--interesting job, married. Then Gemma and Epiphany leave to help John. When they arrive though John has already bested the demon, or so he thinks. The demon comes back and Epiphany sprays it with her concoction. After the demon dies they explain to John what happened and Gemma tells John that it is still his fault, because it was another one of his tricks, that always end up hurting people and this time it was her who got hurt.
Gemma starts sleeping with Terry Greaves to push her uncle into returning to hell to find her mother.