Enzo was first referenced in The Cell, when Damon described being held in captivity with Enzo by the Augustine Society from 1953-1958 in flashbacks. Enzo was described by Damon as a good friend who encouraged Damon not to resent his brother Stefan for not figuring out that he had been taken captive from their family's boarding house.
Enzo and Stefan didn't officially meet until Fifty Shades of Grayson, when Enzo showed up at Whitmore, where Stefan and Damon were holding Aaron Whitmore in order to figure out how to rescue Elena from Wes Maxfield and his experiments. However, Enzo had been sent by Wes to kill Damon in hopes of preventing Elena from being rescued, and he quickly became agitated when his former friend/cell mate and his brother weren't eager to listen to what he had to say. However, Stefan soon parted from the group to rescue Elena himself while Damon dealt with Enzo.
The two did not meet again until Total Eclipse of the Heart, when Stefan returned to the Salvatore Boarding House to find that Damon and Enzo had killed Aaron and made a huge mess of the house. When Stefan confronted his brother about it, Enzo insisted that it was his idea and asked Stefan if he knew where Wes was hiding. Stefan made his displeasure of Damon and Enzo's activities very clear, but Enzo just joined Damon in mocking Stefan and argued that he wasn't interested in being saved. Later on that day, Stefan confronted Enzo in the woods when he was burying Aaron's body, and threatened to kill Enzo if he didn't get out of town, as he didn't think Enzo was a good influence on Damon, especially since he was already upset from Elena breaking up with him. However, Enzo reminded him that because of his decades of being tortured by Augustine, there was nothing Stefan could do to him that hadn't already been done a thousand times before, and added that it would do Stefan good if he just let go of his "do-gooder" attitude. At the end of the night, Stefan ran into him again when he showed up to rescue Jeremy, who had nearly been suffocated to death by Enzo to motivate Bonnie to find a witch to track down Wes for them. Stefan, furious at the lengths Enzo and Damon were choosing to go to get their revenge on Augustine, pointed out that Enzo should have left when he told him to before headbutting him in the face. The two continued fighting until Damon returned to inform Enzo that Liv had done a locator spell and tracked Wes to Richmond.
While Enzo and Stefan did not come in contact in No Exit, Enzo did encourage Damon to call Stefan in hopes that he could rescue them from being locked in a house together by Wes and the Travelers, although this was mostly out of concern for his and Damon's safety more so than Enzo liking Stefan. In While You Were Sleeping, Enzo, who had just found out that the Travelers had the two antidotes needed to cure Damon and Elena of the Ripper compound, informed Caroline that the Travelers needed Stefan to come meet them before they would hand it over. Once Stefan and Caroline met up with him, he explained what had happened since he left, including the fact that Wes had experimented on him in order to create the antidote. He also described what he had learned from the Travelers since he had started spending time with them, revealing that Stefan had another doppelgänger, whom the Travelers needed to die, because the Travelers needed Stefan and Elena to be the only living doppelgängers of their line in order for them to accomplish their goals. Stefan ultimately agreed to do what the Travelers needed to find his doppel, Tom Avery, and trusted Enzo to make sure that Damon and Elena got the antidotes they needed.
Their brief period of civility ended quickly, however, when Enzo was told by one of the Travelers, Sloan that the love of his life, Maggie James, whom he met while being held captive by Augustine, was killed by Stefan in 1960. In Man on Fire, Enzo, in his rage, tracked Stefan down to the restaurant where he was studying with Elena, and angrily explained how he had met Maggie when she was instructed to study his behavior while in captivity. He then revealed that he knew that Stefan had killed her, and tied up Elena and Stefan with help from Liv, whom he had convinced to assist him by threatening her brother Luke. He even went so far as to compel the rest of the restaurant's patrons to ignore them so that he could get the answers he sought. Stefan insisted that it wasn't him who killed Maggie, because he had just gone "straight" after being a Ripper and was barely feeding on people at all, let alone killing them.
However, Enzo had found Maggie's journal of the time after he compelled her to leave Augustine, and it detailed how she had found Enzo's Augustine records and had rediscovered his existence. When Enzo called Damon to confirm what Stefan had done the night Maggie died, Damon, too, insisted that Stefan could have done it, but Enzo simply hung up on him, believing that he was the one who was right. Stefan quickly realized the night Enzo was talking about, and recalled that Maggie had approached him to see if he knew where Damon was, as she had learned about the fire at Whitmore House and assumed that Enzo had perished in it because of Damon's actions. Once Maggie confessed that she planned to kill Damon in retribution, Stefan attacked her with the intent of scaring her out of looking for Damon. Enzo, not pleased with this answer, grabbed Bonnie and threatened her, reminding Stefan and Elena that even though Bonnie was the Anchor, they didn't know for sure what would happen if Bonnie "died." Stefan, not wanting Bonnie to get hurt, lied and claimed that he had, in fact, ripped Maggie's throat out, and that he hadn't given her another thought since, knowing that it's what he wanted to hear. Enzo let Bonnie go and instead started torturing Stefan by stabbing him in the stomach with a stake.
Finally, Damon arrived, and Enzo stopped momentarily to listen to what Damon had to say, which was that he was actually the one who killed Maggie; she had approached him after she approached Stefan, and when she tried to attack him with a vervain-filled syringe with the Augustine crest printed on it, he became thrilled to be able to kill another Augustine member and ripped her head off to make it look like Stefan had done it, since he was still resentful toward his brother at the time for not having realized that Damon had been kidnapped and tortured for five years. He reminded Enzo that his humanity switch had been flipped off at the time, and insisted that he hadn't felt anything because he couldn't bear to feel the guilt and pain of what he had done to him. Furious at Damon but not wanting to hate his only friend, Enzo decided to turn off his own humanity switch, and attacked Liv. Damon and Stefan (who had broken out of his bonds) lunged for Enzo to stop him, but Enzo stabbed Damon in the neck with a bottle and dumped vervain water on Stefan, which injured them to the point that they were too distracted to stop him from vampire-speeding Elena out of the restaurant. Knowing that Enzo would kill Elena to get back at Damon, Stefan and Damon chased after him, and Stefan and Enzo got into a huge fight while Damon left to take an unconscious Elena back to her dorm. Stefan ultimately shoved his hand into Enzo's chest in order to scare him into leaving town and letting go of his grudge against them, but Enzo instead decided to further ruin Stefan and Damon's relationship by forcing Stefan to kill him. He pushed himself off of Stefan's grip, resulting in Stefan inadvertently killing Enzo. When Enzo, in ghost form on the Other Side, realized that Stefan was planning on hiding his death from Damon, vowed to get his revenge on him even in death.