Since I haven't read the 'the last ship novel' in detail, it's hard to say precisely. However, it's likely that there is a lead protagonist, perhaps a naval officer with a lot of experience and leadership skills. There could also be a young and eager crew member who is learning the ropes and provides a different perspective. And there may be a female character who brings a unique balance to the group, whether it's in terms of medical skills, strategic thinking, or just moral support.
I'm not sure exactly who the main characters are without reading the novel. But typically in a story like this, there would be a captain who is in charge of the ship and making the big decisions. There might also be a scientist on board if it has to do with something like finding a cure in a post - apocalyptic world.