Given the title, the main characters are probably the lost girls. They could be girls from different backgrounds, each with their own story of how they became lost. Maybe there's a leader among them or a girl who has a unique perspective that drives the plot forward. It's also possible that there are other supporting characters like people who are trying to find these lost girls or those who are somehow related to the cause of their being lost.
I don't know for sure as I haven't read the book. But it's likely that the main characters are the lost girls mentioned in the title.