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 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.
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, the 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 as well as intensify the love triangle with her, Jeremiah and Conrad.
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 having 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 developed 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 around her, Laurel angrily slaps her. The shock of physically hitting her daughter snaps Laurel back to reality and forces her to realize that Susannah's death has emotionally overwhelmed her as well as blind her to everyone else's grief. After this, she accepts that she needs to learn to properly deal with her pain and become involved with her family and friends again.
Following this, Laurel begins to finally cope with the loss of Susannah. She 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.
Belly brought her over to the summer house when they were fourteen years old for a few weeks. However, her time there proved to greatly hinder the two girl's close friendship. This is due to Taylor's primary focus on "having fun" and flirting with the boys during her stay as opposed to actually spending time with Belly, which hurts the latter girl's feelings. Taylor initially flirts with an uninterested Conrad before setting her sights on getting with Jeremiah, who returns her flirtatious signals to him, and grows to actually like her. Belly makes her promise to "be nice" to him since he has a genuine crush on her. Taylor agrees to be considerate to his feelings.
However, when catching her kissing Steven shortly after this, a furious Belly calls Taylor out on her "slutty" behavior. A hurt Taylor is driven to leave early from her stay at the beach house on her own. It was agreed among the summer group that Taylor shouldn't have come and she remained as a bad summer memory after this. While the girls remained close friends, Belly says they were never the same in their friendship.
Despite this rough patch in their history together, Taylor and Belly still continued to be close to where they still considered each other best friends. They continued to have fights together, which always seems to threaten their friendship, but always get past them. It is implied they continued to remain friends, but maybe not as close compared to their childhood years, in their adult years together.
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. Their father favored Conrad while emotionally mistreating Jeremiah, which they both took note of and held a shared disapproval over. 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 large 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. This repeatedly put their close bond to the test throughout the series.
Jeremiah initially let Conrad have Belly first, but grew sick of seeing him emotionally mistreat her and burying his own feelings. When he finally kisses Belly and begins to make out with her when she returns his affections, a shocked and angered Conrad walks in on the scene. Despite his feelings for Belly, Conrad reluctantly lets Jeremiah have his chance with her as he believes the two will be truly happy together. He then stays apart from both of them for two years after this.
However, in the final novel of the series, the two brother's relationship hits an all time strain and their rivalry with each other hits its boiling point. Conrad still has a strong love for Belly and later admits to hating his brother because of his relationship and engagement with "his girl". Like his brother, Jeremiah secretly resents Conrad because he is aware that he actually still loves Belly and is very wary of his close friendship with her. When Conrad later discovers how Jeremiah cheated on Belly, he is outraged and leaves to calm down. He then confesses his love to Belly, an act which infuriates his younger brother upon finding out about it. This series of events leads directly to their violent and heated confrontation with one another.
They fiercely argue over their rivalry with Belly, fighting about which one of them deserved her. Jeremiah eventually disowns Conrad as his brother, stating that he is dead to him now. The two do not talk for some time after this, but eventually meet up at Laurel's house during the holidays and are forced to talk to one another. Despite it never being directly stated, it is implied the two mend their broken relationship. Jeremiah attends Conrad and Belly's wedding years later, further implying the two eventually made it to good term's again.