There is an intense love triangle between the three "core" characters of the trilogy, that the series is best known for. This is between Isabel "Belly" Conklin and the two Fisher brothers, Conrad and Jeremiah. Readers have their own opinion about who Belly should have ended up with, voicing why Belly was better off with one or the other.
Aside from the love triangle, there are various friendships that are also known to be very strong in the trilogy. Some examples are the sisterly love and friendship between Laurel and Susannah or the rocky but strong friendship between Belly and Taylor.
Below are a list of romances and friendships in the trilogy.
The relationship between Belly and Conrad is known to be the biggest romance in the trilogy. The relationship between the two goes back to when they were both young and children. The two grew up alongside each other since Laurel always brought Steven and Belly to the summer house where they spent their summers there. Belly has a largely known crush on him that started when she was ten years old, which he never returned. Up until The Summer I Turned Pretty, Conrad only saw her as "his best friend's little sister".
This all changes the literal moment once Belly arrives at Cousins in the first novel, fifteen years old going on sixteen and having finally "blossomed". When she arrives at Cousins Beach, both Jeremiah and Conrad immediately see her "as a girl" and no longer as "Steven's little sister".
The relationship between Belly and Jeremiah doesn't officially start or has any true focus on until the second novel, It's Not Summer Without You. During the first novel, Jeremiah, like Conrad, initially did not see Belly as anything more than a close sister-like figure. But he soon develops newfound feelings for her by the end of the first novel. He confesses to her that he now romantically likes her and has hope that she is now over her childhood crush and feelings for Conrad. However, Belly is not sure and leaves him heartbroken. Seeing the connection between Conrad and Belly, he reluctantly steps aside for their happiness.
In the second novel, It's Not Summer Without You, the two become close when Jeremiah enlists her help to find a missing Conrad. The two find Conrad at the beach house and try to convince him to return to college and take his upcoming exams, but fail to do so. They decide to stay with him when learning of Mr. Fisher's intentions to sell the beach house, determined to keep what is left of Susannah. They maintain a good friendship, but Jeremiah still has strong feelings for her and wants nothing more than to be together with her. He is jealous of Conrad and had been furious that his brother decided to suddenly go for Belly despite her well known feelings that everyone was aware of for years.
Laurel and Susannah
The friendship between Laurel and Susannah is noted to be a very strong and sisterly bond. Both women grew up together since they were nine years old. They considered each other "blood sisters" because of how close of a bond they share.
Taylor and Belly
Taylor and Belly have a rocky friendship. They are polar opposites, yet became close friends.
The sibling relationship between Conrad and Jeremiah initially started off as a strong relationship. The two deeply loved their mother, Susannah, yet had different opinions over their father and his mistreatment of them. Despite their different opinions about their father and Jeremiah being furious with Conrad for making their mother worry unnecessarily during her last summer with them, the two still loved each other nonetheless. Conrad had even stuck up for Jeremiah whenever their father emotionally mistreated him when they were children.
However, their shared romantic feelings for Belly that sparked at the same time during The Summer I Turned Pretty proved to drive a wedge between them. Initially, Jeremiah had been willing to let Conrad have her, but soon got sick of putting his personal feelings aside for him. As both brothers' desire to have Belly grew, the three found themselves in a love triangle that intensified over the series. Eventually, Conrad and Jeremiah's rivalry over Belly came to a violent head and both boys viciously fought over who deserved to have her. After this, the two brothers found their bond to be completely destroyed.
However, about a year or so later, they met back up during a family dinner at Laurel's. Although initially awkward, Conrad and Jeremiah slowly mended their broken friendship with each other.