Since I'm not familiar with the 'broken promises novel', the main characters could be people from different walks of life. For example, there might be a politician who breaks promises to the public, a businessperson who fails to keep agreements with partners, and a family member who betrays the trust of their kin.
I don't know as I haven't read it. It could be a male and a female character whose relationship is strained due to broken promises.
There's no way to tell for sure without reading the novel. However, it's possible that there are complex characters. Maybe there's a protagonist who has been a victim of broken promises in the past and is now trying to deal with the new broken promises in his or her life. There could also be an antagonist who is constantly breaking promises to achieve his or her own ends, whether it's power, money, or something else. And perhaps there are supporting characters who are affected by these broken promises in different ways, like friends who are caught in the middle or bystanders who witness the chaos that broken promises create.