The main characters include Hal Jordan, of course. He is the Green Lantern who is going through a rebirth. Sinestro also plays a major role. He has a complex relationship with Hal Jordan, being both an enemy and sometimes an ally. There are also other Green Lanterns like Kilowog who are part of the story. Kilowog is an important figure within the Green Lantern Corps and his presence helps show the structure and camaraderie of the corps. And then there are the villains like Parallax, which is a major force that Hal Jordan has to face and overcome in his journey of rebirth.
Hal Jordan is the lead. He's the one whose rebirth the whole graphic novel is centered around. Sinestro is another key character. His character is very nuanced here, not just a simple villain. Then there are the members of the Green Lantern Corps. They are important as they represent the broader Green Lantern community. For instance, Tomar - Re is there, and his wisdom and experience add to the story. And don't forget the Guardians of the Universe. They are in the background, but they have a big influence on the events in the graphic novel, especially when it comes to the rules and powers of the Green Lanterns.