Erica and Isaac had pretty much in common prior to meeting newly-ascended Alpha Werewolf Derek Hale-- Isaac was lonely and abused by his father in all ways, both physically and psychologically, with his worst punishment being spending many hours locked in a freezer in the basement, which made him develop a case of claustrophobia, while Erica suffered from a severe case of epilepsy since childhood and was bullied by fellow high school students because of it.
However, their lives changed when they met Derek and were given the Bite to gain companionship and power that they never had in their lives, especially Erica who became more sexy after surviving the transformation. They were grateful to Derek and became loyal to their new Alpha, helping him recruit one last Beta, Vernon Boyd, with whom Erica seemed to develop a romantic relationship.
Throughout their time in the pack, Erica and Isaac were sent together to accomplish several of Derek's orders and tasks, including testing Lydia Martin with Kanima Venom to verify if she was the Kanima, breaking into the McCall House to overpower Scott McCall, Stiles Stilinski and Allison Argent and murder Lydia, and distracting Jackson Whittemore, who had indeed become the Kanima, to sedate him and prevent him from killing another former member of the high school's swim team. In time, Boyd and Erica realized that they would not survive the threats of the Kanima and the Argent Hunters in Beacon Hills and left the Hale Pack to search for another one somewhere safer, inviting Isaac to come with them but Isaac prefered to remain in their hometown to help Derek and Scott battle against Gerard Argent and free Jackson from Gerard's control after his first master, Matt Daehler, was killed.
Isaac spent the entire summer helping Derek and the newly-resurrected Peter Hale search for Boyd and Erica after they were abducted by the Alpha Pack and locked in First National Bank's vault for over three months. Unbeknownst to Isaac, Derek or anyone else of the Beacon Hills packs, Erica had been killed by the Alpha Kali when attempting to fight her way out of the vault and Isaac broke into the bank some days later, realizing it was in that bank's vault where Deucalion and his subordinate Alphas were holding his packmates hostage, but he was found, captured and tortured by the Alphas before having his memories of the bank wiped from his mind. He was eventually rescued by mercenary Braeden and found by Derek and Scott. In order to get his repressed memories back, Isaac was submerged in cold water and informed his friends that he saw Boyd and Erica in the vault of First National Bank. He also divulged (though unknowingly) that the Alphas locked him in a closet, with what appeared to be Erica's dead body inside, though he failed to realized that the other Werewolf with Boyd was Derek's long-lost sister Cora Hale.
Isaac and the others received the news that Erica was truly dead when Derek found Erica's lifeless body in the supply closet of the bank. Isaac lamented Erica's death and held a grudge against all the Alpha Pack members, particularly the twins Ethan and Aiden, for her loss.