Theodore "Theo" Merton Jr. is Luc's brainy and pessimistic best friend. He and Luc are the only two people not to be affected by the loop. He is voiced by Kevin Duhaney. In Hebrew he is voiced by Saar Badishi.
Theo is fearful, cautious, studious, and social inept. Deep down, he would have liked to be like Luc and not be seen like a loser by others. However, unlike his friend, who is adventurous, Theo will try to analyze all possible options before making a decision. He always want an answer and a scientific explanation of what happens in the Loop.
He is a 13-year-old African American boy with a thin body, curly black hair and brown eyes. He wears black glasses, a green t-shirt with a radioactive symbol on the front, a pale yellow shirt with elbow-length sleeves that he wears underneath his t-shirt, olive green pants and red and white trainers. He's slightly taller than Luc.
Theo is a pessimistic, phlegmatic boy who is geeky and nerdy. He is not always on board with Luc's ideas because he doesn't just rush through things. He has a quite impressive general culture and knows the logical explanation for pretty much anything that goes on in the loop. Furthermore knows a lot of events about the day they're looped through, backwards and forwards. Though he is not as smart as Gwyn. Also he is cowardly and would rather prefer to take refugee in his own safety bubble of logical facts than to face the society. This make him socially inept.
Luc and Theo are best friends and the only two with knowledge of the loop. They have fun, get into trouble, and take advantage of the loop together. Theo is the voice of reason to Luc, though it usually takes a while before he realizes when he's taking it too far.
Theo loves Gwyn with all his heart, but she does not return the favor. Throughout the series, it shows that Gwyn is Theo’s secret crush, but Gwyn is not attracted by him and instead finds Theo repulsive and avoids him by all means. To add insult to injury, she's also madly in love with his best friend, Luc.
Jesse bullies Theo every day (aside from just the repeating Monday), by putting him in a wooden box and farting into it.
- Theo has a single father, named Theo Merton Sr., who only appeared in the episode "Badmerton to the Bone"
- He and Luc's phones match their respective shirt colors. Theo's phone and shirt are green.
- His name is Theo Zweistein in the German dub.
- In "All Things being Sequel" he's erroneously called Theo Matthews.
- His voice actor also voices Cameron from Total Drama.