Person of Interest gave Reese (Jim Caviezel) his first and only love interest when it introduced Iris (Wrenn Schmidt), but the show abandoned their romance shortly before it began. After forming a romantic relationship, the two received very little screen time together and Iris was written off shortly being introduced.
Dr. Iris Campbell was a therapist that Reese met while working under his new identity as Detective John Riley in season 4. After getting into trouble for shooting too many criminals, Reese was ordered to undergo therapy sessions with Iris. She ultimately fell in love with Reese after hearing him discuss his issues for a few episodes, and the two started seeing each other. But of course, Iris knew nothing about who he really was. In Iris’ only season 5 episode, Reese broke up with her because he knew what the fight with Samaritan would mean for his future and that it would only put her in danger. This marked Iris’ last appearance on Person of Interest.
How Person of Interest ended the Reese-Iris storyline felt quite abrupt, considering that, in the season 4 finale, when Reese was worried about his cover being compromised, he told her to go into hiding and that he would tell her everything later. This led to an expectation that Iris could finally find out about Reese’s true identity and his association with the Machine, but this conversation that Reese promised never took place. Based on comments from Jim Caviezel, it would seem that their breakup was rushed because season 5 was shortened. Caviezel has said the low episode count meant romantic relationships would be limited to just five minutes [via Give Me My Remote].
This falls in line with Person of Interest’s stance on love stories, considering the writers and producers typically avoided romantic subplots with its main characters. Much of what went on with Root (Amy Acker) and Shaw (Sarah Shahi) on-screen was just tension, and it wasn’t much different for Reese. The show didn’t even imply a possible romantic connection between his character and Carter (Taraji P. Henson) until their unscripted kiss during her death scene in season 3. The reason why Person of Interest broke its romance rules with Iris in the first place is because the show wanted to spent a season exploring whether or not Reese could finally have a personal life, which is something he wasn't allotted in the first three seasons.
Iris fit perfectly into Reese’s season 4 story, but that’s not what the final season was about in terms of his character arc. The show’s final season focused on bringing Reese’s mission to a close, since the endgame was for Reese to die saving Finch. To get Reese closer to his season 5 ending, it was important for Reese to break off his relationship with Iris and accept that to do what needed to be done, he had to give up on having a normal life. Though Iris’ screen time on the show after the two ended their therapy sessions and began a relationship was significantly reduced, her character did bring something interesting to the table in season 4. Iris’ scenes with Reese led to deep, revealing conversations about his outlook on life and his struggles with moving on. It was disappointing that Person of Interest didn’t have time to push this further.