Isaac Colbert is Emily's ex boyfriend.
Isaac is tall and thin, with light blue eyes.
Isaac attends a Catholic high school, Holy Trinity Academy, and is the lead singer of a local band named Carpe Diem. He and Emily first meet when he's performing at Holy Trinity Church one evening. While he's singing, he notices Emily in the crowd and chats with her after the band finishes their performance. He invites her to go sledding and the two exchange numbers. They find that they have a lot in common and Emily questions her sexuality even more. They have a great evening of sledding and talk about their family lives a bit. Emily hesitates in telling him about her bisexuality. They get together again at a restaurant but her ex Maya shows up unexpectedly. Before she can say anything to the two, Emily jumps up and runs off, leaving Isaac very confused.
At the Hastings fundraiser, Emily finally tells Isaac about her sexuality. To her surprise, he isn't bothered by it at all and wants to continue dating her. In "Killer" the two become a serious item and confess their love for each other. They lose their virginity to each other at Isaac's house and they are nearly caught by his mother. Isaac isn't aware of the rift between Mrs. Colbert and Emily. When his girlfriend tells him about it, he dismisses her story as a lie and argues that his ex lied about his mother's hostility as well. He discovers that Emily was telling the truth when he finds a picture of them in the kitchen drawer. It was from a school dance but his girlfriend's head was cut out of the photo. After Isaac confronts his mother, he apologizes to Emily at the opening of the Radley Hotel. Unfortunately for him, Emily is still upset about his curt dismissal of her accusation and she breaks up with him. She later gave birth to their daughter and put her up for adoption without telling him anything.
Issac later reappears in Emily's life leading to debate after getting closer to him again if she should tell him about their daughter, but can't get the courage to. Later on Emily is forced to tell him and he walks away not wanting to deal with her, but he later texts her saying "I want to talk". Emily is not sure if this a good or bad thing.