Most of the characters are drawn directly from Fitzgerald's own life:
- Amory Blaine—the protagonist of the book, is clearly based on Fitzgerald. Both are from the Midwest, attended Princeton, had a failed romance with a debutante, served in the army, then had a failed romance with a second debutante (though after This Side of Paradise's success, Fitzgerald won back Zelda).
- Beatrice Blaine—Blaine's mother was actually based on the mother of one of Fitzgerald's friends, rather than his own.
- Isabelle Borgé—Amory Blaine's first love is based on Fitzgerald's first love, the Chicago debutante Ginevra King.
- Monsignor Darcy—Blaine's spiritual mentor is based on a Monsignor Fay, to whom Fitzgerald was close. Fay was from Minneapolis.
- Rosalind Connage—Amory Blaine's second love is based on Fitzgerald's second love, Zelda Sayre. Unlike Zelda, Rosalind was from New York. Rosalind is also partially based on the character Beatrice Normandy in H. G. Wells's novel Tono-Bungay.
- Cecilia Connage—Rosalind's cynical younger sister.
- Thomas Parke D'Invilliers—one of Blaine's close friends (also the fictitious author of the poem at the start of The Great Gatsby) was based on Fitzgerald's friend and classmate, the poet John Peale Bishop.
- Eleanor Savage—a girl Amory meets in Maryland. They share their love for literature and fall for each other during the summer. But they break up after Eleanor sends her horse off a cliff and nearly dies herself.
- Clara Page—Amory's widowed cousin, whom he loves. But she doesn't love him back.