As I haven't read the book, it's hard to determine the exact main characters. However, based on the title, the 'good luck girls' are surely important characters. There could be a leader among them or some with distinct personalities. There might also be characters like an old seaman who could have some knowledge about Shipwreck Lane and play a role in the girls' story. Or perhaps a local store owner who provides a connection to the community in which the girls live. Without reading the novel, it's all speculation though.
Since I haven't read the novel, I can't say for sure who the main characters are. But it's likely that the girls referred to as the 'good luck girls' are the main focus. There may also be other supporting characters like people who live in Shipwreck Lane or those who interact with the girls in some way.
I don't know as I haven't read it. But I assume the main characters are the 'good luck girls' mentioned in the title.