Steven Conklin is Belly’s brother and the son of Laurel. Steven is a good friend to Conrad and Jeremiah and a supportive brother towards Belly. Though we see little of him only in flashbacks, overall, his truthfulness is a springboard for Belly to make difficult yet wise decisions. He mainly appears throughout Belly's flashbacks making fun of her, often humiliating her.
He is absent for most of the series, taking off to look at colleges with his father in The Summer I Turned Pretty and making only short and brief appearances since then. The only way we get most of his character is through his apperance in Belly's summer memories and flashbacks. In the first novel of the series, in one of Belly's many flashbacks, he has a brief fling with her best friend Taylor. Belly catches the two kissing despite the age gap (Steven being around 15 and Taylor 13). He regrets it, however, and the incident has never been brought up since.
Despite his regular teasing towards his younger sister, it is stated and shown that he has a strong protective side of her. This in which is supported by Jeremiah who says that Steven, despite his teasing, is "really protective" of Belly. He even says to Belly herself in We'll Always Have Summer that while Jeremiah was like a brother to him, Belly was his "little sister and [she] came first", something that clearly surprises her. During a dinner in celebration for her and Jeremiah's upcoming wedding, he tells Jeremiah to "treat her right" as "she's the only sister [he's] got". This statement causes Belly to get emotional as she's never heard him say such a thing about her before.
The only novel he is a supporting character in is the final novel, We'll Always Have Summer, wary of his little sister's choice to marry so young, but still supporting her. He cheers up a bit once hearing Jeremiah wanted him and Conrad to be his best men. When Jeremiah is nowhere to be found before the wedding, Steven says that he is going to "kick his ass" for doing this to Belly, who is getting worried.