Leah was created by writer John Ney Reiber and artist Peter Gross in The Books of Magic issue 6, in 1994.
Major Story Arcs
Leah has been a succubus for at least thousands of years, going from master to master. She was originally an angel, and was convinced to rebel against God by Lucifer. She lives in a small jewelry-sized box and eats doves. She wants to be free. As a succubus, she has no soul, but thinks those who do have one are the crazy ones.
She shows up in Tim Hunter's school under the identity of a girl from California. Her looks and short skirts get all the boys' attention, including Tim's--right when he has started a relationship with Molly.
Her current master is Martyn, one of the only remaining members of the Cold Flame, a group of evil sorcerers. Martyn wants to control Tim and his magic, and uses Leah to manipulate him.
Leah starts flirting with him, and Tim responds to a degree, but he still feels strongly about Molly. Molly, however, is quite annoyed with Leah for what she considers her more obvious charms.
After Martyn magically burns Tim's dad Bill, he sends Leah over to comfort him. However, Molly gets there first, and Leah can tell that she's a good person, and is doing good things for Tim, who she has started to legitimately like. However, Tim, angry about his dad, acts out against Molly, who runs off. Now Leah is happy to manipulate Tim after all.
She gets Tim to stay at her and Martyn's house. Tim finds himself infatuated with her. However, Martyn ends up getting killed by Barbatos and the Sir Timothy from the future, and Tim escapes the house. In the process, he takes over Martyn's golem, making him into his own golem, Happy. Leah is impressed at the power Tim has, and at his spirit.
With Martyn dead, Leah is free, and also frees the doves she's supposed to eat. However, she could still be summoned by someone new. She grabs Martyn's car and drives Tim to the hospital to see his dad.
She tells him how he can sacrifice a cat to save his dad, but he won't do it, further impressing her. She asks Tim to hold on to the box she lives in, choosing her own master for once. He agrees, but then uses his power as her master to set her free forever.
Girl in the Box
Leah moves to California and starts a successful career as a teen model. When Tim comes to America to visit Zatanna, she accidentally runs into him at the airport.
Tim and Molly have been somewhat on the outs (she is currently stuck in Faerie), and Leah is glad to have a chance to be alone with him, but also still respects Molly.
She gets Tim to come with her and her model friends back to their agency. Two kids, Oscar and Herschel, who want to be big-time, see them and try to impress them while driving, but fail. Leah kisses Tim to distract the boys, who crash into a sign. Tim doesn't know what to make of this.
The boys hook up with Psyche and Cupid, the Greek gods, who have remade themselves as gangsta rappers. They go to Leah's agency to get revenge. Cupid shoots Tim and Leah with chocolate bullets to make them fall in love with each other.
Leah and Tim laugh it off, however. They both know what real love is, and it can't be faked. Tim defeats the two gods and gets them to accept a better way of living in the modern world.
Tim and Leah go camping in a part of the desert called Little Moon. Meanwhile, the Keeper of the Tide, a mystic being, bursts out into normal reality, in the same desert, from her sacred plane.
Tim and Leah find her. She is in mermaid form, and is dying of dehydration, tangled up in the remains of the fence that she knocked down earlier when she entered the earthly plane.
Leah and Tim can't figure out why there's a mermaid in the desert, and can't move her--her body is like water. Leah moves their camp near her, as Tim is entranced by the topless mermaid's assets, leading Leah to realize that Tim hasn't slept with Molly yet.
They manage to put her in a tarp and fill it with water, waking her up.
The dying mermaid cryptically implies to Tim and Leah that she doesn't live on actual water, but the depths of the soul.
Tim misunderstands and thinks she drinks blood, but Leah says that's wrong, and in fact Leah is the one who used to be a "blood-thing." Now she just starves herself.
Tim and Leah fight. She is angry that he sees the mermaid as such a different kind of being, and is so disconnected from her, but Leah feels very connected to her as another eternal being.
Leah is afraid Tim looks down on her as a succubus, and Tim is afraid because he has no experience with girls, especially now that he has feelings for both Molly and Leah. She and Tim admit that they like each other.
They go to bed separately, and in peace, but Leah can't resist. She likes him too much, and makes herself look like Molly. She lays down with Tim and they make out, and are about to have sex, when Tim realizes she's really Leah. He's mad at her, but as he says, he's especially mad because he doesn't know what he would have done if she had come to him just looking like herself.
They kiss. Their conjoining brings down a magical rain, created by Tim as an Opener, which starts the desert to flowering. The mermaid is now cured, and sits in their convertible, now full of life's water.
They all talk. The mermaid doesn't understand why they can't control their magic and their reliance on words, while they are not sure they can trust her, as an elemental. The mermaid splashes them, and they all disappear from the real world, and Tim and the mermaid reappear in her tower of the Moon, a vast unending space of water.
The mermaid tests Tim's understanding. A swarm of naked water spirits come out of the moon and swirl around him, teasing him for his half-boy, half-adult state, his continuing embarrassment over sexuality and nudity, his fear of "swimming with" Leah, and the fact that he "has never felt the tide." However, one remonstrates the rest, saying that perhaps Tim is waiting for his "Other," a term the original mermaid also used. But then she looks deeper into him, and says it was just his fear, after all. The water spirits leave, and Tim is unmanned.
Outside the Tower of the Moon, Leah and the mermaid watch Tim's travails and Leah yells at the mermaid for torturing him. The mermaid says this is necessary for him to grow up. She says she is the last Keeper of the Tide, and is alone in the Tower, dying. Only Leah's dreams of her love for Tim are keeping the mermaid alive now.
A Storm of Chaos starts to rain down inside the Tower. The Keeper makes a chalice and tells Leah she must drink it, to become the new Keeper, to understand her own life and cycle and tides, to stop Chaos, and to save Tim. The Keeper dies.
Leah doesn't want to drink the cup filled with the spirit of the mermaid. Her whole life has been about trying to escape a box and getting to be herself, and if she drinks, she'll once again be imprisoned and forced to live under another's ideas. But, to save Tim, she drinks it.
Leah wakes up as a mermaid, and despite her former fears, loves her new reality. She sees the relationships between everything, which are symbolized by two fish swimming in a bowl, named Yin and Yang.
Tim has gone through more experiences of self-awakening himself, and is now comfortable with his feelings for Leah. He waits for her, to take their trip to see the Sequoias.
Leah, however, as the new Keeper of the Tide, is now stuck in her Tower, and can no longer even see Tim. They are separated forever.
Much later, Tim is skating along, feeling free of much of what has recently happened in his life, when he sees Leah's box on the ground in a parking garage. He checks it out, and is sucked in. Leah has intentionally trapped him to teach him a lesson, which seems quite out of character for how they last left things, and also because she is not supposed to be able to leave the Tower, but in fact picks the box up in the garage in the real world.
Tim spends years in the box, and has to come to terms with himself. When he's an old man, and about to die in a fire, Leah shows up in the box and saves him, but also castigates him for never looking for her in the desert. But she accepts that he has changed, and lets him out.
Much later, Tim's Other, one of the beings he created by accident through his imagination and difficult feelings as a child, has been destroying all the other universes Tim made. He also collects some of the beings from those worlds, which are versions of the real people Tim knows. Various versions of Leah as a mermaid are members of the Other's collection.
After Tim defeats the Other and Barbatos, Leah is seen still happily watching over the forces of chaos and order (as incarnated in the fish Yin and Yang), and the cosmos, as the Keeper. She is the last person seen in the series other than Tim.
In addition to the other versions of Leah that the Other carries around in a box, the Other meets her a few times on various world's Tim created. These include a version that sleeps with the Other by mistake, and a member of the Teen Titans-esque group the Mystic Teens. Leah is Moonchild, similar to Raven. She is the sidekick to that world's Phantom Stranger. She reveals herself to be working with the Other, and sabotages the other Youth. She was mad at this world's Tim and wants the Other to love her now. However, the Other kills them all.
Powers and Abilities
As a succubus, Leah can do basic magic like create new clothes for herself, and shapeshift to look like other humans.
As the new Keeper of the Tide, Leah has some degree of omniscience, and is able to watch over the world. She presumably has some degree of reality-warping or mind-illusion powers, like the previous Keeper.