The main character in Dear Mr. Henshaw is Leigh Botts, an inquisitive, troubled boy who writes letters to a children's author, Boyd Henshaw. Mr. Henshaw is a character in the story, but he never actually comes on the scene. All we know about him is what Leigh tells us and the letters he writes.
Leigh's parents are also important characters. Bonnie is Leigh's mom, and she struggles to raise Leigh as a single mom. Bill is Leigh's dad, who left the family, and who Leigh doesn't get to see very often.
At school, his teacher is Miss Martinez, but the person who encourages Leigh the most is the custodian. The librarian is Miss Neely, who notices Leigh's talent for writing. Angela Badger is another character who is a children's author who Leigh gets to meet. Angela Badger calls Leigh an "author" and gives him a sense of worth as a writer.