Well, The Walking Dead started as a comic series. But often, multiple issues of the comic are compiled and republished in a bound format, which is called a graphic novel. So, it's kind of both.
Yes, it can be. It can serve as an introduction to Negan's character, which is a significant part of the 'Walking Dead' story. Even new fans can quickly get into the story and understand his role.
Rick Grimes is a central character. He is a former sheriff's deputy. He's tough and resourceful, constantly striving to keep his group alive in the post - apocalyptic world. His leadership skills are crucial for the group's survival.