Barry Allen and Iris West are the main couple of "The Flash". Following the murder of his mother and his father's arrest, Barry went to live with Iris and her father and ever since, they grew up together and were each other's best friends. However, unknown to Iris, Barry has always been secretly in love with her, but has never had the courage to confess how he truly feels. When Barry becomes the Flash, he strongly wishes to tell Iris the truth about his identity, but is forced to keep it a secret to protect her. During this time, Barry is also upset when Iris begins a serious romantic relationship with her father's detective partner, Eddie Thawne. Throughout the first season, Barry struggles with his feelings and dealing with his newfound powers. Eventually, Barry gathers the courage to confess his feelings to Iris during Christmas, but she rejects him. Barry and Iris's relationship becomes awkward and they work on slowly resorting their friendship.

Barry attempts to move on from Iris and later begins dating reporter, Linda Park and in the process, Iris finally realizes her true feelings for Barry. Before Central City is attacked by a tsunami, Iris confesses her feelings for Barry and they share their first kiss and in the process, Iris finally learns Barry's identity. Barry manages to save the city when he discovers his ability to travel back in time and manages to prevent the attack on Central City. However, the timeline is altered and thus preventing Iris from realizing her true feeling and she continues her relationship with Eddie, much to Barry's heartbreak. However, hope for Barry and Iris's romance is confirmed when Barry discovers that in the future, he and Iris will be married.

Towards the end of the season, Iris finally learns the truth about Barry's identity and although she is hurt, she becomes a member of Team Flash and later gets engaged to Eddie. Unfortunately in the season finale, Eddie sacrifices himself to save everyone, leaving Iris heartbroken by his death. In the second season, Barry and Iris's friendship is restored as Barry begins a new relationship with Patty Spivot and Iris continues to mourn the loss of Eddie. Throughout the second season, Iris slowly begins to move on and finally begins to realize her true feelings towards Barry at last when she begins to realize and acknowledge the signs they are destined to be together. By the end of the season, Barry and Iris share another kiss, but the timeline is once again altered when a distraught Barry changes the timeline and prevents his mother's death.

In the third season, Barry briefly lives a new life in the altered timeline, but later realizes his mistake and restores the true timeline. Finally, Barry and Iris at long last become a couple and they develop a strong and serious romantic relationship and later move into together. Iris also becomes a more active member of Team Flash and later become co-leader. However, Barry has many struggles throughout the season as he deals with the fallout of his messing with the timeline where several changes have been one, including his future with Iris. Barry makes a horrifying discovery that Iris will die in the future and is determined to save her. Barry and Iris later get engaged, but later realize it wasn't out of the best reasons, learning it was one of the changes Barry makes to the timeline to save Iris. Due to this, the couple take a break from their relationship to reconsider everything, but shortly after, later reconcile in a special musical crossover with Supergirl. Finally the remainder of the season focuses on everyone's efforts to save Iris and they manage to succeed. However in the season finale, Barry and Iris are separated when Barry must travel into the Speed-Force to restore balance.

In the fourth season, Barry and Iris are reunited at last after Barry is freed from the Speed-Force. Although Barry and Iris deal with their separation issues, their relationship remains strong and they finally begin planning their wedding. Halfway into the season, Barry and Iris at last become husband and wife after an epic four-part crossover event and get married alongside Oliver and Felicity. However, their happiness is shortlived when Barry is framed for murder by their newest enemy, DeVoe, but after some time, they managed to prove him innocent and defeat DeVoe by the end of the season. However, they make a shocking discovery when they meet their daughter, Nora who has traveled back in time from the future. In the fifth season, Barry and Iris deal with meeting their daughter and the strained relationship between Nora and Iris. Eventually, they all managed to become a family and work together, but everyone is utterly shocked upon the discovery that Nora is secretly working with Barry's arch-nemesis, the Reverse Flash.