The main characters might include those who are directly affected by the events leading to the last days in California. This could be a family who has lived in California for generations and now has to face the end of what they knew as home. There could also be some sort of hero or anti - hero figure, someone who either tries to save California or is contributing to its downfall.
I don't know as I haven't read the book. It could be a variety of characters, perhaps ordinary people trying to survive the last days, or it could be politicians or business leaders who are in a position to either make things better or worse.
There's no way to be certain without reading the novel. However, it's likely that there are characters representing different aspects of California society. Maybe there's an environmentalist who is fighting against the forces that are causing the last days. Or a young person who is trying to find their place in a world that is falling apart. There could also be a group of people who come together to face the challenges, and within that group, there would be various personalities and motives.