"Just out of curiosity... Do you remember the other night?" "You mean the night before last?" "That night, before you... weren't you." "Yeah, I remember..." "So, that night... were you you, or were you not you?" "You mean, the night when we were us?" "Yeah. I just wanna know if... if it was actually you with me." "...Did you want it to be someone else?" "No! No. No, of course not." "Good. Because it was me. And I do remember it. I really remember it."
— Allison and Isaac's final conversation in Insatiable
Allison and Isaac's relationship had a great deal of development throughout their time on the series. Though Allison helped Scott McCall, Stiles Stilinski, and Derek Hale save Isaac from being killed by an ArgentHunter in Shape Shifted, the two eventually became enemies due to the fact that, at the time, their respective packs (the precursor to the McCall Pack for Allison and the Hale Pack for Isaac) were rivals. Their relationship only became more antagonistic at the end of Season 2 when Allison-- who was devastated by the death of her mother, Victoria Argent and who was manipulated by her grandfather Gerard Argent into fighting against the Beacon Hillswerewolves-- attacked both Isaac and Derek using her Chinese ring daggers.
This tension between the two persisted through the first few episodes of Season 3A until Isaac caught Allison helping Scott, Derek, and himself to capture the full-moon-crazed Vernon Boyd and Cora Hale in Fireflies. Several days later, Allison and Isaac both got lunchtime detention and were forced to work together to restock the supply closet, during which time Allison apologized for stabbing him during the battle in Master Plan. When Isaac then had a panic attack as a result of Ethan and Aiden locking them in the supply closet, Allison was supportive of him, even after he accidentally hurt her in the process, leading the two to forgive each other for the past and start to begin an uneasy friendship.
Throughout the rest of Season 3A, Allison and Isaac's friendship grew closer and closer as they worked together to try to solve the Darach sacrifice murders while also helping Scott and Derek defend themselves from the Alpha Pack. By the midseason finale, it was clear that both Allison and Isaac were beginning to develop romantic feelings for one another, which was complicated for both of them due to their relationships with Scott, who was Allison's ex-boyfriend and Isaac's best friend and soon-to-be Alpha. In Season 3B, the two began exploring their romantic feelings more, especially after Scott began a romantic relationship with Kira Yukimura and essentially gave them his blessing to be together. After attempting to deny their feelings for one another, they finally shared their first real kiss in Silverfinger and began their romantic relationship at that point. They went on to have sex for the first time in De-Void, though the fact that Isaac was possessed by a Nogitsune fly at the time concerned Allison, as she was unsure if it was truly Isaac who she had slept with and if it was what Isaac really wanted.
Unfortunately for both of them, their relationship met an abrupt end in Insatiable when Allison, Isaac, and Kira were forced to engage in battle with the Oni. Isaac was eventually overpowered by one of the Oni, who was about to kill him when Allison stepped in and shot the Oni with a silver-tipped arrow, which instantly killed it, distracting Allison long enough for the Oni to stab her in the abdomen and kill her right in front of Isaac. Isaac was subsequently so devastated by Allison's death that, after he and Allison's father, Chris Argent, helped the McCall Pack defeat the Nogitsune and the Oni, he decided to join Chris and leave Beacon Hills in favor of moving to France, where he apparently remains to this day.
Allison and Isaac are also known as Allisaac by fans.
Throughout Teen Wolf
In Shape Shifted, though Allison and Isaac didn't physically interact, Allison was instrumental in the plan to help save Isaac from being killed by a Hunter. She overheard her father, Chris, and her grandfather, Gerard, talking about how they planned to kill Isaac, as they believed him to be the person who killed his father, Mr. Lahey. After informing Stiles Stilinski that the Hunter planned on killing Isaac with a lethal strain of purple wolfsbane, Allison followed the Hunter with her compound bow and quiver of arrows, first shooting the tire on the Hunter's car to delay him and then shooting him in the leg to slow him down further. However, after that, she was unable to help Stiles and Derek Hale rescue Isaac further because she had to help lock Scott McCall up for the full moon.
In Ice Pick, Chris Argent, Bennett, and some of their fellow Hunters captured Allison at the gas station and brought her to the Hale House ruins. After tying her up and tricking her into believing that Chris had also been captured with her, Chris confronted her about the fact that she shot a fellow Hunter with an arrow to prevent him from finishing his mission. When Allison reminded him that he was going to kill Isaac, Chris insisted that Hunters make difficult decisions like this every day.
In Venomous, upon learning about the Hale Pack's suspicion that Lydia Martin was the Kanima, Allison, Scott, and Stiles set in motion to keep Isaac and Erica (who had orders from Derek to test, and, if necessary, kill her by exposing her to Kanima Venom) from getting near her. Despite their best efforts, Isaac was still able to coat Lydia's rock candy with the venom, and when Lydia failed to be paralyzed by it, the Hale Pack set off to kill her before the Kanima could wreak any more havoc. Allison, Stiles, and Jackson Whittemore, wanting to protect Lydia from Isaac and his pack, coerced her into coming to the McCall House under the guise of having a study group, believing that they would have a better chance of defending her there.
When Allison peeked out the window and saw Isaac, Erica, Derek, and Vernon Boyd standing across the street and watching them, she called Scott, who had been held back by Coach Finstock, to tell him to come home quickly to help defend Lydia as she pulled out her mini-crossbow. When Isaac was the first to infiltrate the house through the back door, he came upon Allison and Stiles, who managed to help subdue him with Scott's timely intervention. Allison also was able to incapacitate Erica by coating her crossbow bolt with Kanima venom she found on a windowsill in Scott's room, allowing Scott to throw the unconscious Isaac and the paralyzed Erica at Derek's feet outside. It was then revealed that it was Jackson, not Lydia, who was truly the Kanima.
In Master Plan, Isaac and Scott ran into Allison's fatherChris while trying to sneak Jackson Whittemore's body out of Beacon Hills Memorial Hospital, and Isaac listened as Chris insisted that he was losing Allison and that he knew Scott was losing her as well. The three worked together to bring Jackson's body to the Warehouse District to meet Derek and Peter Hale, where they were ambushed by Allison and Gerard. After the newly-awakened Jackson, in Kanima form, threw Derek through a wall, Allison attempted to shoot at Scott, but when he ducked, the crossbow bolt instead shot Isaac in the shoulder. Isaac managed to pull out the bolt, allowing him to heal quickly enough to wolf out and join Derek and Scott in fighting the Kanima. Allison lunged at Isaac, who attempted to punch her with both hands, but she easily ducked to avoid his hits before stabbing him in the abdomen and the back with her Chinese ring daggers multiple times. After the battle ended and Gerard, who forced Scott to make the paralyzed-Derek give him the Bite to cure his cancer, began rejecting the transformation by bleeding and vomiting black blood while Allison, Isaac, and the others watched in horror.
Allison and Isaac's actors, Crystal Reed and Daniel Sharman, were in a romantic relationship in real life for around two years until they broke up in 2013.
Isaac was the second person and werewolf Allison had a relationship with in the series, with the first being Scott McCall. Additionally, Allison was Isaac's first and only romantic partner who was shown in the series, though he has shown romantic interest in several characters, such as Lydia Martin, Erica Reyes, and Cora Hale.