Theo is played by actor Cody Christian.
Officially, Theo's sister, Tara, died of exposure after being lost in the woods 8 years ago but Theo was apparently with her and did nothing as he watched her suffer with a broken leg in a freezing creek.
He claimed to return to Beacon Hills with his parents specifically to join Scott's pack, but it his true plan was to take away Scott's pack and his Alpha status.
Stiles found portions of Theo's story don't add up, specifically his father's signature on a school form seems to be forged. Theo's "parents" seem terrified of him and he is seen preparing to crush his father's hand with a hammer to explain away Stiles "evidence" against him.
Lydia, through her visions, learned that Theo killed his sister on purpose so that the Dread Doctors could transplant her heart into him making him a natural chimera.
However, his plans to kill Scott failed, so he settled for stealing La Bête du Gévaudan's power with a pack of chimeras. Deucalion, under the pretense of helping Theo, manipulated him to killing his remaining pack members for their power and not use Belasko's talons to siphon the Beast's power.
Realizing he was tricked he attempted to kill everyone, but Kira Yukimura opens a hole in the Earth and his dead sister dragged down into the hole before it closed.
Theo joins Scott and the gang as they settle in to read "The Dread Doctors" novel. He kills chimera Josh Diaz when he attacks Stiles. Theo then blackmails Stiles to keep quiet about the killing. (Read More...)
Theo and Stiles watch over the vet clinic waiting to see who comes to take the chimera's body. Theo saves Stiles from a fire. (Read More...)
Theo reveals himself as a chimera and tries to take Scott down on the night of the supermoon. (Read More...)
Dr. Valack says that experiments on Theo marked a turning point in the Dread Doctors' research. Theo continues to entice Stiles onto his side. In the future, he breaks into Eichen House for Lydia Martin. (Read More...)
He helps his pack develop their abilities and devises ways on how to control them. (Read More...)
Theo uses the Dread Doctors' technology to help Malia find her mother. He double crosses Braeden and Malia, and The Desert Wolf hands him a jar containing the glowing talons from Creatures of the Night as payment. (Read More...)
Theo and Stiles search for Lydia in the tunnels of Eichen House. He leaves with Valack's Dread Doctor helmet in hopes that it will help figure out who the Beast is. (Read More...)
After killing Josh and taking his power, Theo works along side Scott and Liam in order to find Mason and steal his power as well. (Read More...)
After killing Tracy, Theo shows up in the tunnels and takes out Scott, Liam and Lydia. He catches up to Sebastien and tries to take his powers but he fails. He is double crossed by Deucalion, and his dead sister drags him down into the earth. (Read More...)
Liam releases Theo from the ground for his power. Theo restrains Liam against a wall, demanding to know where his sister is. Liam and Hayden remind him that she's dead. It is discovered that he no longer has the power he absorbed from Tracy and Josh. (Read More...)
Theo suffers a recurring nightmare, each one ending exactly as the last, with Theo's sister tearing his heart from his chest. The cycle continues until he tells her it's okay, she doesn't have to stop. (Read More...)
Theo used chemicals from the Dread Doctor's lair to resurrect four of the failed chimera lying dead at the Nemeton. After he revived them, he said they are his pack and expected them to follow him.
While all of them had enhanced strength and were immune to Mountain Ash, each pack member had unique abilities of their own.
The pack dissolved when Hayden and Corey defected, and Theo absorbed the spark of power from both Josh and Tracy, killing them in the process.
- Tracy Stewart - Kanima Hybrid
- Corey - Chameleon Hybrid
- Josh Diaz - Unknown Hybrid with electrokinesis
- Hayden Romero - Werejaguar Hybrid
- Deucalion - Werewolf
- “You don't remember me, do you? I guess I look a little different since the 4th grade.” — Theo to Scott McCall in Creatures of the Night
- “You know Stiles, I came back here for Scott but I also came back for you. Someone like you, someone whose willing to walk into the woods in the middle of the night to protect his friends. I don't have anyone like that, Scott does, you all do. I know I'm in the right place, I'm meant to be here, I'm meant to be apart of this pack.” — Theo to Stiles Stilinski in Dreamcatchers
- “Let me help, it doesn't have to mean I'm part of the pack, like you've excepted me or anything like that. It just means I can help catch this girl.” — Theo to Scott McCall in Parasomnia
- “Scott, I came here hoping to find a pack, I wasn't planning on watching one fall apart.” — Theo to Scott McCall in Required Reading
- “A werewolf's eyes are suppose to change color when you take an innocent life.... these look blue to you? It was self defense, for me and for you.” — Theo to Stiles Stilinski in Strange Frequencies
- “I'm the first chimera, its the coyote part you don't notice. Its why Malia trusted me first even though she probably didn't know it. You found the perfect word though Scott.. because a chimera isn't just a monster with different parts, it can also mean something impossible to achieve, an unrealizable dream.” — Theo to Scott McCall in Status Asthmaticus
- “I never lied about why I came to Beacon Hills, I'm here for a pack. I came for the werecoyote, the one whose first instinct is to kill. I came for the banshee, the girl surrounded by death. I came for the dark kitsune. The beta with anger issues. I came for void Stiles, that's the pack I want. Unfortunately, it doesn't include Scott.” — Theo to Stiles Stilinski in Status Asthmaticus
- “I know you saw the fresco, two seriously pissed off creatures, the hellhound and the beast fighting over a pile of dead bodies. I don't want to be one of the bodies, its that simple. I can get you to Lydia or we can see who gets to her first.” — Theo to Stiles and Scott in Amplification
- “Yeah, I was nine years old, I also believed that a guy in a red suit came down the chimney to deliver presents. So when three people in leather masks showed up and said that my sister wanted me to have her heart, I believed them too.” — Theo to Stiles Stilinski in Lie Ability
- “I found Deucalion. You're right, I'm going to help Parrish stop the beast. I'm going to take its power and then I'm going to break its neck. So maybe I'm not the good guy in your eyes, but I might end up being the guy that saves all your asses.” — Theo to Stiles Stilinski in Lie Ability