The main character is Greg Heffley. His family members like his parents and his brothers also play important roles in the story. And of course, his friends at school are main characters too as they are part of his daily experiences.
The main plot is about Greg Heffley's experiences in middle school. He has to deal with all kinds of typical middle - school problems, like friendship and family issues. In 'The Third Wheel' story, there are also events related to school dances and trying to fit in with his peers.
It's not historical fiction. The book mainly presents the typical life and problems of a current-day youngster, rather than having any historical context or elements.