Leo was a child of Dionysus born into a wealthy family. Every one of his relatives held respectable jobs, such as lawyers, doctors and business owners. His mother had broken the trend, becoming an insanely popular model and falling into heavy drug use. She had a one-night stand with Dionysus and thus Leo was born.
Leo had no interest in having a full-time job and family. He was a party animal just like his parents. Eventually the drug use took its toll and after a close encounter with death from an overdose, he decided to go clean. Jaded and cynical about the world, he bought a farm in country England, about 2 1/2 hours south of London, rejecting all of his old friends and habits. He bought horses and greenhouses and spent his days tending to the animals and gardening. He got so obsessed with the latter that eventually he had a ridiculously large array of flowers, rivalling collections in the nicest gardens in the world. Eventually he made a business out of it, with tourists paying to enjoy his land. Persephone was drawn to his passion for nature and visited him one day. He knew who she was and was enthralled by her, falling head over heels for the goddess.
When she left him he was distraught. He closed down his gardens, leaving all of the flowers to die, and fell back into his old drug habits. When he received Mallie, their daughter, he was simply pissed off. He tried to raise her at first, but realising he was terrible at it, hired a nanny to do it for him.
Mallie's childhood was filled with half-assed training from her father, caring for the horses and schooling from the nanny. Growing up, she was always very lonely. There wasn't a lot of things to do on the farm and turned to art as a way to escape.
Her first monster attack came one day when she was roaming around their land on horseback. Her father had always been open about her parentage and so she was not surprised, only scared, when she spotted the Hellhound. She saw the creature and screamed, her horse getting spooked and running back in the direction of the house. She alerted her father and the two of them managed to fight it off.
After the initial terror, monster attacks were never much of a problem for Mallie because of her father's skill in battle. In fact she actually looked forward to them because at least they were interesting. Her father however hated them and suggested she go to camp. As much as she wanted out of the countryside, going to a camp in America did not sound like fun to her. She wanted to see the exotic places in the world, not be stuck with a bunch of teenagers. Mostly, she didn't want to listen to her father.
So she stayed at the farm until she was just shy of 17 and then decided it was time to leave. She told her father she was going to camp, so he gave her access to her trust fund and she got the hell out of there. She travelled the world, passing through as many countries as she could. She financed her travels with her father's money and odd jobs such as teaching english, working as a nanny, and as many random things she could do.
She moved around so much that monster attacks weren't such an issue and when they did happen she managed to fight them off with her father's training. As much fun as she was having, travelling didn't satisfy her as much as she thought it would and she fell heavily into drugs. After loosing all of her money, she contacted her father and came clean to him about her lies and the life she was living. As much as he was angry at her, she was still his daughter and so he paid for her to go to rehab in Thailand. When she was officially clean, but still broke, he suggested she go to camp. She begrudgingly agreed, and arrived shortly after by plane, mostly just to see what it was about.