In the 'Golden Boy' novel, the main character would be the 'golden boy' of course. But there could be other important characters as well. His parents might play a significant role in shaping who he is and his upbringing. There could be a best friend who is always by his side through thick and thin. And then there might be a character from a different background who challenges the 'golden boy's' world view and forces him to grow and change. There could also be a teacher or a coach who has a big influence on his development.
I don't know for sure since I haven't read it. It could be the 'golden boy' himself as the main character, and perhaps his friends, family members, or rivals.
Most likely, the titular 'golden boy' is one of the main characters. He might be surrounded by people who either support him or are jealous of him. There could be a mentor figure who helps him along his journey, or a love interest who adds another dimension to his story. Maybe there's also an antagonist, someone who tries to bring down the 'golden boy' for their own reasons, like envy or a desire for power.