In May 2022, Dex Heller was revealed to have filled in for Chet Driscoll as a temporary manager at Volonino's gym while Chet was out of town. Chet hired Dex because they were both military veterans. When Chet returned from his trip, Dex expressed an interest in working for Sonny Corinthos' criminal organization. Sonny was initially reluctant, but Dex was able to show that he could handle working for the mob.

It was soon revealed that Dex was working undercover for Michael Corinthos. Michael had recruited Dex from a private security firm to spy on Sonny and gather enough proof of Sonny's illegal activity to put Sonny in jail for life. Sonny was slow to trust Dex, and Sonny's doubts were reinforced when Sonny's cousin Brando Corbin expressed reservations about Dex. Sonny asked his tech and surveillance expert to dig into Dex's background. Brick uncovered that Dex had been dishonorably discharged from the Army for assaulting a superior officer. Sonny confronted Dex about the incident, prompting Dex to open up about his past.

Dex explained that he'd been tormented and bullied throughout his childhood by his two older brothers, and his brothers had put him in the hospital five separate times. Dex also revealed that his parents had brushed the violence off as "boys being boys," so Dex had enlisted in the Army to get away from his dysfunctional family. According to Dex, his commanding officer had been toxic with the soldiers and abusive with the man's own family, so Dex had given the commanding officer a taste of his own medicine. Sonny empathized because Sonny had suffered terrible abuse at the hands of his own stepfather.

A few weeks later, an unknown person wielding a hook attacked Ava Jerome during a picnic at the Quartermaines'. Josslyn Jacks found Ava barely clinging to life. Dex heard Josslyn's cry for help. As Josslyn watched, Dex stanched the bleeding and performed CPR until paramedics arrived and raced Ava to the hospital. Josslyn was impressed with Dex, and she began to see him in a different light.

After Brando Corbin was attacked and killed by the hook-wielding maniac, suspicion briefly fell on Dex. Furious, Sonny arranged for his men to kidnap Dex. Dex was taken to Pozullo's and hanged from a meat hook in the walk-in freezer. Dex denied being the killer, but Sonny arranged for his henchmen to torture Dex. Luckily for Dex, Josslyn found and rescued him before he could be seriously hurt or killed. Dex was quickly vindicated when Diane was attacked while Dex was locked in the freezer.

Dex was more determined than ever to help Michael take Sonny down, which meant that Dex had to assure Sonny that Dex didn't hold the incident against Sonny. Sonny assigned Dex to help Anna Devane escape from police custody when she was framed for a murder that she hadn't committed. Dex posed as a police officer and drove the prison transport van that had been scheduled to take Anna to Pentonville. When Dex missed a turn, the police officer riding with him realized that Dex was not a police officer. During a brief scuffle with the police officer, while Dex was driving, Dex was shot, and the van crashed.

After Anna successfully escaped, Dex made his way to Michael's home, but Josslyn was there babysitting. Dex was forced to ask for Josslyn's help, and he guided her on cleaning and stitching up the gunshot wound. Eventually, Josslyn took Dex to her dorm room to recover, but he developed an infection. Josslyn reached out to her mother for antibiotics, and she nursed Dex back to health. Dex and Josslyn grew closer, and they shared a passionate kiss. Unaware of Dex's secret mission, Josslyn implored Dex to walk away from Sonny, but Dex refused.

Things heated up when Sonny was approached by a group called Pikeman Security, who wanted to move arms through Sonny's territory. Valentin Cassadine acted as an intermediary between Pikeman and Sonny, and he promised that Sonny would be paid well. Dex told Michael about the agreement, and both Michael and Dex agreed that it was exactly what they needed to take Sonny down.

In January 2023, the Hook Killer struck again. This time, the target was Josslyn, but Britt Westbourne managed to intervene. Dex appeared on the pier and shot the Hook, who fled into the night. Josslyn wanted to call the police, but Dex explained that he'd shot the killer with an illegal gun. Britt assured both Dex and Josslyn that she would handle the police, and she told them to leave. Neither Dex nor Josslyn realized that Britt had been nicked by the poisoned tip of the deadly gaffing hook that the killer used. Britt died from the poison.

Dex and Josslyn returned to his apartment, where they made love for the first time. They kept their romance under wraps because Josslyn had just ended a long-term relationship with Cameron Webber, and Dex and Josslyn knew that Sonny would never approve. Cameron learned the truth about Josslyn's romance with Dex when he caught them together at her dorm, but he agreed not to say anything. A few weeks later, Dex took a bullet for Sonny when an assassin went gunning for Sonny. Dex was shot in the shoulder, and he recovered at Sonny's penthouse.

In late March, Josslyn learned about Dex's arrangement with Michael when Michael stopped by Dex's apartment. Michael was not pleased that Dex was involved with Josslyn, but Dex and Josslyn refused to end things. Instead, Dex and Josslyn agreed to keep their distance until Dex had completed his mission.