|“|| "I told you we'd end up on the same side."|
"How about I punch you in the balls to remind you we're not?"
"I love this kid."
— Theo and Liam in The Beast of Beacon Hills
Theo introduces himself to Malia, Liam and Kira as a former classmate of Scott and Stiles after he helped Scott defeat the Chimera Belasko in Creatures of the Night. Theo secretly was allied with The Dread Doctors who turned him into a Chimera when he was nine or ten years old as Theo had killed her sister to get her heart. Theo's Chimera nature wasn't revealed until Status Asthmaticus, as Liam was worried about Hayden dying from her mercury poison in Lies of Omission, Theo manipulated him to kill Scott (as Liam was the only one that could steal Scott's True Alpha status) and he could take the status to himself from Liam, as Mason frees Liam from his anger of the supermoon to go see Hayden, Theo angered kills Scott himself but he knew that would ruin his chance to become an Alpha Werewolf, as Parrish came to take Hayden to the Nemeton, he gives her a last kiss.
After the events of the supermoon, Liam started to blame himself for allowing Theo manipulate him to kill Scott. Then even they're enemies, he and the rest of his packmates allied with Theo and his Chimera Pack to invade Eichen House to retrieve Lydia. As Theo failed to absorb the Beast's power thanks to Deucalion that was working with Scott the whole time. Theo tried to kill him as he helped kill the Beast but Kira uses her sword to summon Theo's sister, who drags him into a sinkhole where his status remains unknown.
Three months after being send to the ground, Theo is released from his prison by Liam and Hayden, who needed the eletromagnokinetic powers he stole from Josh to help them lure a Ghost Rider. Theo, once freed, attacked Liam and started asking to the couple where his sister is. They explain he killed her years ago. After talking, they met with Garret Douglas who recognized Theo and who Theo recognized being Der Soldat. Theo grabs a lightning rod and gets violently shocked. Its revealed he lost the powers he stole from Josh and Tracy. Liam and Hayden are surprised when Theo remembers Stiles (maybe due to being underground he still remembered everyone he knew before they being erased). Scott finds out about this and says that Theo can't be trusted due to the fact he killed him, the other chimeras and his sister. Liam explains that using Theo might be a mistake. Beside that, Noshiko Yukimura trusted Liam to hand him the sword and everything Theo does, it will be his responsibility. Liam later broke Kira's sword as Theo promised to tell him, Mason and Hayden everything he knows about Garrett Douglas if they break the only way to send him back underground. Later in the hospital Theo and Liam fought Ghost Riders after Sheriff Stilinski is also erased. They manage to get the upper-hand but more riders come. Theo decides to be the bait, he tells Liam to go out of the hospital and is presumable he was erased as well
Throughout Teen Wolf Edit
In Creatures of the Night, as Theo came to help Scott and Kira, Liam along with Stiles and Malia appears too. As Belasko is defeated and runs away, Theo introduces himself and he says he heard that Scott had become a True Alpha.
In Lies of Omission, as Theo says to Scott that Stiles killed Donovan on purpose (as he's lying to separate the Pack), he helps Liam and Scott fight the Doctors to protect Hayden but Hayden gets injected with mercury anyway.
In Status Asthmaticus, as Theo is separating even more Scott's pack, he manipulates Liam to kill Scott but as his plan fails he kills Scott himself to save Hayden, but even he wasn't an Alpha, he used a special serum to resurrect Hayden ,Tracy Stewart, Josh Diaz and Corey as his pack.
In Codominance, Scott travels to Shiprock, New Mexico with Stiles to get Kira and her mother and told Liam to do not do anything until he's back. As he sees Theo with Hayden in the school, Mason warns him to do not get involved. As Scott returns with Kira and Noshiko to Beacon Hills, Liam and Mason got an information from Corey that Theo is looking for a blind alpha and Scott says that must be Deucalion.
In Amplification, after hearing Scott's plans of getting Lydia out of Eichen House from Corey, Theo decides to help the McCall Pack with the Mountain Ash barriers and he actually also wanted Lydia so he could get to Parrish who could guide him to The Beast.
In The Beast of Beacon Hills, after killing Josh, gaining his electromagnokinetic powers and finding the identity of The Beast, Theo allies with Scott and Liam to find Mason, who was captured by The Dread Doctors, he reveals to them that The Doctors used a Nazi Alpha Werewolf called Der Soldat which means German for "the soldier" to expand their lives. After arriving to The Doctors' new lair, they find Mason hooked to the tube of Der Soldat and The Doctors appeared. The Surgeon calls Theo a failure for repeating the same mistake again and again and says he and his comrades learned from Theo that true evil only comes by corrupting something truly good. He fights The Dread Doctors and Liam joins him but Mason then turns into The Beast and reaps apart the Doctors.
In Apotheosis, after killing Tracy and gaining her Kanima powers, Theo procides on his final chance to absorb The Beast's power. He shocks Deucalion, Scott and Liam using Josh's power and uses the Kanima Venom to paralyze Scott. As he fails get The Beast's power he's shocked that Deucalion was working with Scott the entire time and gets his neck snapped. After The Beast is killed and Mason is saved, he tries to kill them all for ruining his chance to get the power and Kira uses her sword to summon Theo's sister and the spirit drags Theo into the sinkhole Kira created.
In Ghosted, Liam and Hayden were losing options how to lure a Ghost Rider by absorb enough lightning. Liam proposed use Josh because he ate electricity but Josh was dead and Kirs left to train with the Skinwalkers. The couple had to get Theo back from his prison and use the eletrokinetic powers he stole from Josh. After getting Kira's sword from Noshiko Yukimura, Liam uses it to free Theo, who climbs up from the hole and partially shifts before attacking Liam. Theo asks where his sister is, they explain he killed her. Theo angrily proclaims that he's going to kill them all. Liam instead of send him back to the prison, explains what Kira's sword can do. Theo met with Garret Douglas in another part of the tunnels and Theo grabs a lightning rod and gets shocked. Liam and Hayden realize he doesn't have Josh and Tracy's powers anymore which led him to say he did just put them back the way he found them. He asks for Scott and Stiles which surpises both Liam and Hayden since Stiles was erased from their memories but not from Theo's. Later in Scott's house Theo explains they can't bring Stiles back from the Wild Hunt, he explains the Riders take someone and go away with that person. Scott arrives home and is shocked to see Theo in his kitchen. Malia also comes in, partially shifts and attacks the chimera.
In Heartless, after Malia beats Theo. Scott and Liam talk to each other. Scott says that Theo can't be trusted. He reminds his Beta that Theo manipulated him in order to kill Scott but he did that himself and reminds that he killed Tracy, Josh and his sister Tara as well. Liam says that using Theo might be a mistake but Scott also did mistakes while learning to be an Alpha.If they want Stiles back they have to use everything they have. They create a trap for a Ghost Rider in the Dread Doctors' lab in the woods. Parrish arrives a few minutes later and tries to free the rider but Scott and Liam manage to stop him and Theo is able to restore the mountain ash barrier to prevent the rider from escaping. Garret Douglas arrives to the lab and shifts to his werewolf form and threats Theo to tear him in half if he doesn't break the barrier. Theo, for his life, breaks it and Douglas kills the rider with just one hand and his powerful fangs and eats its pineal gland.
In Blitzkrieg, Liam, Hayden and Mason are in the Sheriff's station with Theo locked in a cell. Yhey talk about Mr Douglas being able to kill a Ghost Rider. Theo proposes to tell them everything he knows about Douglas if they break Kira's sword because its the only way to send him back underground. Liam breaks it and they start to hear Theo. Theo explains that Douglas is actually a Löwenmensch, a creature half wolf, half lion, and says that he and other soldiers confronted the Ghost Riders once in the woods and the riders use their guns to erase the soldiers and before escaping Douglas was hit by their whip. To heal his wound, Douglas met the Dread Doctors but instead of helping him they hooked Douglas to a tube and used his healing factor to expand their lives for the followed 70 years and now Douglas wants the Wild Hunt as his own army. A Ghost Rider arrive and erases Mason which causes Liam to fight for the loss of his best friend but Hayden decides to let them take her as he wants Liam to keep with Scott.