So bare with me, this may be a lot to read, but this is a plot I've been craving so badly! Also, this is being written super quickly, so sorry if it's a bit messy.
It would be set in a sort of medieval period, where four royal families live in the same kingdom, and fight each other for dominance and the throne. At the moment, the current Queen and King in power are cruel and ignore those of lower class. Due to this, the poor struggle day to day for food and basic medical supplies. They live in a city-like structure, where the castle is on the top level, and the further down you go, the more poor the people are. The city/kingdom is 6 levels in all, the elite and rich living between 5-6. My character would have grown up on level one, abandoned at a young age and therefore became a bit animalistic in nature while trying to scrounge for food. One day, she went as far as stealing from others, then she went further and killed. At a young age she was caught by the guards and sent to a dungeon on level two where she met your character. While there, she was picked on a lot and beaten up. She developed a hate of people in general, and when her guards and other patrons attacked her and sexually assaulted her at night, she began to hate men even more.
Now, your character came in and saved her from one such attack, by fluke, and she instantly became attached. Basically, she sees your character as the only good person in the world and began following him around everywhere. Eventually the two of them worked out a plan and escaped, running from the kingdom to live in the forests surrounding it. From there, they built up a little gang of rebels who hate the royal family in power. There's many such gangs hanging out in the forests, but yours gets a pretty nasty reputation over the years.
Your guys, while his history is up to you, I do have a vision for. He should be possessive, ruthless in his killing, he's been a bad guy through and through. She's basically his pet and he more or less treats her like one. He sends her on missions, pushes her around and is abusive on some fronts, but he does have a certain fondness for her as well that he just doesn't like to show cause it's a weakness to him. She will seriously do anything for him, even hold back her anger and jealousy when she sees him with other girls or if she finds out he's sleeping with them more than her. I imagine him as being very prideful, an excellent fighter, and someone who seems apathetic for the most part until you anger him, then he's ruthless and doesn't hold back.
Basically the RP would be following them, their struggle for power among the other gangs, their overall goal to take down the queen, developing their relationship, etc. If you have any questions, feel free to shoot them at me.