Here are some of the main characters in Grit:

Angela Duckworth: The author, who developed the concept of "grit," which refers to people's ability to persevere after setbacks. She is a former management consultant and public school teacher. Duckworth studied positive psychology with Martin Seligman at the University of Pennsylvania. She decided to study what contributes to people's success and came up with a scale that measures people's grit.

Martin Seligman: A professor of positive psychology at the University of Pennsylvania and Duckworth's advisor in graduate school. He developed the idea that learned helplessness leads to clinical depression, and he developed Cognitive Therapy, which operates on the premise that people's thoughts can shape their response to suffering and that they can develop optimism, partly through their thoughts about what happens to them.

Carol Dweck: A psychologist whose work Duckworth discusses in her book. Dweck has come up with the idea of a "growth mindset." This is the idea that people are not static in their abilities but that they can change. Her ideas run contrary to those of many educators, who have traditionally looked at IQ scores and other measures and decided what students are capable of. Instead, Dweck believes that if educators and students believe that they can continue to grow and change, they will be capable of more.

Mihaly Csikszentmihalyi: A psychologist who came up with the idea that if people are really absorbed in what they are doing, they exist in a state of what he called "flow." People may be absorbed in difficult work, but if they are in a state of flow, they are able to persist.

West Point cadets: An important part of the beginning of the book. Duckworth describes their grueling summer course, called The Beast. For many years, people have tried to figure out why some cadets make it through the course, while others don't, as they are all well qualified and hand selected. Duckworth figures out that grit explains why some make it and some don't.