Last Updated September 5, 2023.
The Introduction to the Principles of Morals and Legislation by Jeremy Bentham is a work of non-fiction, so characters don't appear in it in the same way they appear in a novel. That doesn't mean there aren't any, though. In fact, it's the people who are the most important actors in the treatise. It's fascinating, it's a seminal work of Western political thought, and you should read it. You should also check out the excellent study guide available on this website.
The "characters" in Bentham's work are politicians and, by association and according to his beliefs, the people who elected them. They should, according to the Principles, try to achieve a balance between good and bad outcomes when electing people and making laws. Bentham called this "utilitarianism," or the principle of using laws to achieve the greatest good for the greatest number of people. This tradeoff between "pleasure" and "pain" is the bedrock principle in the Principles.
This means that the people are more important that the laws, ideas, or structures of society. That's not the case in other works of political theory. In Leviathan by Thomas Hobbes, for example, people are secondary, and the form of government, monarchy in that case, is paramount. In the Principles, the people are the most important because moral trade-offs are involved. You can make laws to cope with the consequences of moral choices, but you cannot create the choices themselves out of nothing. People must do that. That's the essence of Bentham's work. It's an argument about moral choices. Thus, it's an argument about people.