Obviously, the slave girl herself would be the main character. She could be from any species in the Star Wars universe, like Twi'lek perhaps, known for being slaves in some parts of the galaxy.
The main character, the slave girl, might be someone who has been in slavery for a long time. She could have unique skills or knowledge that she has gained during her time as a slave. For example, she might be an expert at sneaking around the slave - owners' compounds, which could come in handy later in the story when she tries to escape or help others escape. It's also possible that she has some sort of connection to an ancient power or artifact in the Star Wars lore, which makes her story even more interesting.