Quentin Coldwater (Jason Ralph) starts out as a bored high school senior who finds himself accepted into a magical college, Brakebills. While there, he learns that Fillory, a kingdom featured in his favorite children's books, is real.
With his friends, he travels to Fillory, and together they become kings and queens. In the books, his main love interest is Alice, who dies at the end of the first novel, but is later brought back. Their relationship ends when Quentin cheats on Alice in a threesome with his best friend Eliot and Janet (Margo in the show). Alice and Quentin begin to rekindle their romance in season four of the show.
Eliot flirts with everyone, including Quentin. The two are close friends, and at various times in the books, their friendship feels romantic. Quentin sometimes seems jealous when Eliot flirts with other men, but other than the threesome, nothing concrete materializes. However, the show takes the implied romance much further. During a quest, Eliot and Quentin end up living their entire lives together in an alternate timeline while attempting to solve a puzzle. Early on in their time together, Quentin makes the move and kisses Eliot.
(Spoilers ahead)
Upon their return to the correct timeline, Quentin calls their time together "proof of concept" and suggests that they try to have a serious romantic relationship. Eliot spurns him, but comes to regard his choice as the greatest regret of his life. Season 4 ended in a shock to fans as Quentin was killed off, with season five dealing with the aftermath of his death.