To know Marley's story one has to go back a bit further, back before she was born, back to her grandfather. Her grandfather owned a very lucrative company that produced outdoor gear, as well as offering the actual outdoor experience by heading up anything from kayaking trips to mountain climbing expeditions. Her grandfather loved the outdoors, but his true passion was mountain climbing, which he did every chance he got, and always challenged himself to climb bigger and harder mountains. This was precisely how he met Cybele, one one such mountain climbing expedition, there was a rock slide, injuring some of the others in his group. Cybele watched from afar as the man made trip after trip, in dangerous terrain to save the others. His love for nature and mountains had her smitten, and she eventually found a way to meet him, a brief affair pursued, and as all god and mortal stories end, she left him with a son.
Eventually he married, and the step mother cared for the son as if it were her own. Once he hit 12, all the normal demigod things happened, which I won't bore you with those details, but one very important detail, he wasn't quite like other demigods. On top of his ADHD and Dyslexia that most demigods have, he was diagnosed with severe OCD and they suspected Asperger's as well, though the tests were inconclusive. Eventually things got so bad, he was put on medication, which helped him to grow up and lead a relatively normal life.
At the age of 19, he was asked by some friends at camp to help with a quest that involved trekking into some dangerous mountain ranges. The quest leader was a child of Eris, during the trip the child of Eris fell and if it hadn't been for the father's quick thinking, he would have died. Eris was grateful, and after the quest started hanging around the father, and eventually a short affair pursued. Months later, a little baby girl was left on his door step. He wasn't really sure what to do, but he knew he couldn't give her up, so he moved in with his family, and decided to raise her with their help.
When she was only 2 years old, tragedy struck, the father's parents were killed in a plane crash. Between the grief of losing his parents, and the stress of raising a child, he went off his much needed medications. It wasn't long before he was quite "off his rocker", to the point he took his daughter, and essentially ran away into the wilderness. Being so far away from civilisation and technology it was easy to avoid monsters for the most part. Marley was raised with no idea what technology was, and most of what she learned growing up were survival skills. Things like, trapping, hunting, fishing; even her communication skills were minimal, as her father rarely spoke in full sentences unless it had to do with her training.
For years this went on, though she had some typical Eris traits, being raised so far from anywhere, she was far from a typical child of Eris. She also had no idea who her father had been when he was in civilisation, let alone that he was quite rich after inheriting billions. Eventually he had to tell her the truth, that she was a demigod, and that so was he, at least he had enough sense to do that. He trained her as best he could when the occasional monster showed up. Things seemed as if they could go on like this forever, that is, until a Drakon showed up when she was 16.
If it hadn't been for a group of nearby huntresses they both probably would have died, as it was the father died saving her life. In a moment of clarity just before he took his last breath, he gave her a key, he apologised for not having told her the truth before. The key went to a safe located in his bedroom. She had always wondered what was in it, but stopped asking years ago. The huntresses talked her into going to camp where she'd be safe, so she went to pack some belongings. Upon opening the safe, she finally learned the truth about who her father was, about the millions that was his, and that was now hers.
So with some belongings, and the paperwork from her father's safe, she went with the huntresses to camp, where she begins a long journey of discovery.