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.
Romances
Belly and Conrad
The relationship between Belly and Conrad is perhaps the biggest romance in the trilogy. The relationship between the two goes back to when they were both young children. The two grew up alongside each other since Laurel always brought Steven and Belly to the summer house to spend their summers there. Belly had 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 moment Belly arrives at Cousins in the first novel, having "blossomed" as a beautiful teenage girl and he take notice of her "as a girl" for the first time.
Belly ends up starting a brief relationship with Conrad, only for it end on a rather bad note. Following this, despite both appearing to still have feelings for each other, the two have a rocky and unclear relationship. Conrad struggles with his negative emotions regarding his family drama, which strains his relationship with Belly, who grows frustrated with not knowing his true emotions or feelings.
Belly and Jeremiah
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. Jeremiah, like Conrad, did not see Belly as anything more than a sister-like figure. However, he had a very strong friendship and bond with her as the two grew up together as children. When Belly finally "blossoms" as a teenage girl, he develops newfound feelings for her and wishes to be together with her. When realizing her lingering feelings for Conrad, Jeremiah finds himself heartbroken and longing to be with her.
However, two begin spending significant time together during It's Not Summer Without You. Due to Conrad's confusing signals regarding his feelings for her, Belly finds herself pulled towards Jeremiah and begins to realize that she may like him after all. This only serves to complicate their relationship together as well as intensify the love triangle with her, Jeremiah and Conrad.
Friendships
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 both children and developed a strong friendship. Despite no biological connection, they considered themselves "blood sisters" based on the close bond they shared with each other. The two knew everything about each other and when Susannah develops life-threatening cancer, it takes a large toll on Laurel's emotional state. She tries to argue with Susannah on how to handle her cancer, which briefly strains their friendship.
Upon her eventual death, Laurel is perhaps the one who has the most trouble acceping her best friend's death. She becomes almost all consumed by her grief, even forgetting about her children's own suffering from the loss as well. When Belly calls her out on only focusing on her own grief and forgetting about everyone else, Laurel angrily slaps her. The shock of hitting her daughter causes Laurel to realize how Susannah's death has emotionally overwhelmed her and that she needs to learn to properly deal with her pain. Following this, Laurel begins to accept and cope with the loss of Susannah. She then decides to look after her late friend's sons, Conrad and Jeremiah, in order to honor her best friend's memory.
Taylor and Belly
Taylor and Belly are best friends since they were kids in school. However, despite their close friendship, the two have a rather rocky relationship together. This is likely due to them being polar opposites from each other personality wise - with Belly a shy tom boy and Taylor a flirtatious girly girl. Despite their tough times in their friendship, both girls remain close through their adult years together.
Siblings
Conrad and Jeremiah
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 and looked out for one another.
However, their shared romantic feelings for Belly proved to drive a wedge between them. As both brothers' desire to have Belly grew, the two found themselves in a rivalry with each other that intensified over the series and puts their close bond to the test.