Rex

 He's got a truck. He used to date Ellie's sister. Nick's step brother. He's older. He's in college.

The guy she decided to be her first on an unexpected whim. Possibly for all the wrong reasons. It wasn't what she expected. Since that time, after a long riff, he wanted to get back together 2 summers ago. He drove all the way to Texas to find her. But that wasn't enough.

Last time she saw him, he'd met someone that he goes to college with.

He gave Nick trouble when Nick hooked up with Heath, but he's been rather aloof lately.