Nathan was first introduced to the series when Kelly was working in a dating agency. He was one of her clients, and she was interested in him then, but wasn't allowed to date him.
He returned to the series later, after Kelly had finished with the dating agency and her network marketing 'cult' (as Evan referred to it) and started seeing her.
It didn't take very long for Nathan to admit his love for Kelly. However, they have had a number of difficulties. Nathan is Jewish and his felt his parents want him to marry a Jewish girl, but Kelly isn't Jewish. Kelly seemed to take more notice of this when her boss pointed out there might be a problem and she began looking into it, much to the dismay of Nathan.