Nathan is Alex's neighbor in her apartment building. When Alex becomes a full-fledged agent of Division, she got an apartment as part of her cover, where she meets Nathan. There's an instant attraction between the two, although Alex is reluctant to pursue anything because of her cover.
Alex started up a platonic relationship with Nathan when she was invited to his party as a welcome-to-the-building gesture. After an obnoxious man tried to dance with her when she repeatedly refused, she physically assaulted him and left the party feeling embarassed. Soon after, she and Nathan got past this event and became friends.
In The_Next_Seduction Alex invites Nathan over for dinner, and Michael creates an uncomfortable situation where he implies that Alex is for sale. Although Alex is humiliated, Nathan still expresses interest in her and they kiss.
Eventually, Nathan and Alex begin a romantic relationship, where Alex demonstrates in her dreams how much she desires to have a normal life with Nathan and perhaps even start a family.