While there are many characters in The Summer I Turned Pretty series, the main "core" characters of the trilogy are Isabel or "Belly" Conklin, and brothers Conrad and Jeremiah Fisher.
Isabel "Belly" Conklin - The main character, narrator and protagonist in the series. She is the daughter of Laurel and younger sister of Steven. She is known to be persevering, head-strong, sometimes domineering, and yet equally fragile. She has had a crush on Conrad since she was young, but her relationship with him proves to be rather rocky throughout the trilogy. This is further complicated when she finds herself developing feelings for Jeremiah, resulting in an intense love triangle between the three. She also holds a passion for swimming and the feeling of being in water, finding it peaceful, and holds a great talent for the sport.
Conrad Fisher - The eldest son of Susannah and Mr. Fisher and older brother of Jeremiah. He is known to push away his loved ones and keeps his emotions and feelings inside and rarely, if not ever, expresses them. He’s rebellious, stubborn, and infuriating a great deal of the time with his family and friends since they struggle to understand him and his actions. In the first book, his character is something of an "asshole" to everyone, having spent the summer getting drunk and locked in his room. It is suspected, however, that he has developed feelings for Belly due to his sudden jealous and over protective attitude towards her when other boys display an interest in her. Despite everything, he actually loves his family and friends and wants only what is best for them.
Jeremiah "Jere" Fisher - The supposed golden boy of the Fisher family and supportive force in the family where Conrad is known to be the turmoil. He is a good friend towards Belly and a loving brother to Conrad and deeply loves his family more than anything. Like Conrad, he falls for Belly in the first novel but allows his brother to pursue her first. However, he grows tired of burying his feelings and demands his fair chance with her. By the third book, his character goes through an ugly transformation from Golden Boy to Frat Boy and even cheats on Belly with the "sister sorority slut". He proposes to Belly to prove his guilt, but is shown to have dramatically changed for the worst.