|“|| "Tell me about my son. Tell me one thing about Stiles that you remember. Just one."|
"You swipe that keycard, I'll tell you anything you want."
"[...] One thing. Just one thing!"
"He was smart! Smart enough not to trust me."
Theo and Stiles were forth grade classmates until Theo and his family left Beacon Hills after the death of his sister Tara in the early 2000s. In the autumn of 2012, Theo returned to Beacon Hills, claiming that he had heard that Scott McCall had ascended to the rank of True Alpha and he wanted to join the McCall Pack, and leading them to believe that he, too, was a recently-turned Werewolf. However, Stiles was immediately suspicious of Theo's motives, as he found the fact that Theo returned at the same time that the Dread Doctors started creating Chimeras to be too convenient to be a coincidence.
However, Theo was dedicated to his portrayal as a well-intentioned Werewolf, and Stiles' investigation into Theo's shady plans went nowhere. Additionally, the supernatural drama occurring in Beacon Hills forced Stiles and Theo to have to work together, and the numerous good deeds Theo performed, such as saving Lydia Martin's life when she was slashed by Tracy Stewart's Kanima tail and preventing the electromagnetokinetic Chimera Josh Diaz from uncontrollably killing Stiles on the roof of Beacon Hills Memorial Hospital, caused Stiles to begin to let his guard down around Theo.
Unfortunately for Stiles, Theo was soon revealed to be a Werewolf-Werecoyote Chimera and partner of the Dread Doctors who used the secret that Stiles was forced to kill Donovan Donati in self-defense to start breaking the bonds between Scott and Stiles, as well as Scott's bonds with the rest of the pack. Theo's ultimate goal of stealing Scott's Alpha powers (by having Liam Dunbar kill Scott) and then taking the McCall Pack for his own, though this plan was eventually thwarted when Theo, in a fit of impulse, killed Scott himself, preventing him from becoming an Alpha. Stiles was especially horrified when Theo revealed that he wanted Void Stiles in his pack and insinuated that the fact that Stiles had more blood on his hands then any of them made him more like Theo than Stiles wanted to admit.
Despite the disaster Theo wrought on the McCall Pack, they had no choice but to align themselves with Theo and his new pack of Chimeras in order to fight the even greater threat Beacon Hills was facing, the recently-resurrected Beast of Gevaudan and to save Lydia Martin from her torturous stay at Eichen House. However, this time, Stiles and his pack knew better than to trust Theo, and at the end of the season, they successfully imprisoned Theo in an underground cavern with help from Kira Yukimura and the Skinwalkers.
In Season 6, Theo was released from his prison by Liam Dunbar and Hayden Romero, who sought to use his powers to help fight the Ghost Riders. When Theo asked them where Stiles was, they realized that he, unlike the rest of them, had his full memories of Stiles, and used them to help the pack fight against the Wild Hunt, assisting Liam in buying enough time for Scott, Malia and Lydia to try to open a rift to bring Stiles back from the Phantom Train Station and possibly hinting at Theo's potential redemption and reconciliation with Stiles.
Throughout Teen Wolf
- Theo was notable for specifically wanting Stiles and Malia to be in his pack, believing that, since they had both killed before, he had the best chance of tempting them into embracing their darker nature just like him.
- For this reason, Theo encouraged Donovan Donati to try to kill Stiles in order to get back at Sheriff Stilinski, knowing that Stiles would likely be forced to kill Donovan in self-defense, which would not only put a wedge between Scott and Stiles, but would also make it that much easier to tempt Stiles into embracing his violent side.
- Theo once told Stiles that he reminded him of his sister, Tara. Whether or not this is a true statement or something he just said to try to win Stiles over remains to be seen.