Theo lives in the same town as the hero, Moki Town. The main character has known him since they were both young. It is revealed at Professor Bamb'o's lab that Theo always followed the hero around while they were growing up. There is an interesting contrast between the player character and Theo. While the hero's father essentially abandoned his child after his wife disappeared, Theo's father raised him with a very protective style of parenting. (Theo's mother is absent for unexplained reasons; however, post-game conversations imply that she may have died when Theo was young.)
If the hero loses against Theo when they get their starters, Theo still goes home to his father, but this time he is overjoyed and brags about the whole battle to Cameron. Cameron then asks the hero if they lost on purpose or not.
The hero runs into Theo periodically during their journey, though Theo is always one step behind.
Spoiler warning! This article contains plot details about events that occur later in the game.
After the meltdown of Tandor's first power plant, Theo fears for his father's life. He is with the hero during the attack on the second power plant, and escapes Vinoville with the player on the back of a Pokémon Ranger's Staraptor. He vows to complete the gym challenge, although the stress of worrying about his father is continually bothering him.
The player encounters him training on Maskara Island after the 6th gym, where he is somewhat more reserved than usual. He can be rebattled endlessly until the player challenges the 7th gym.
At the Gym in Snowbank Town, Theo has a change of outfit and seems more confident and mature. Since the gym challenge requires two people to take it together, he teams up with the player for the Five Trials. He and the player challenge the Gym Leader together in a double battle, after which point they both receive Mega Bracelets and the power of Mega Evolution. He then challenges the player on top of the mountain.
After defeating the 8th gym in Tsukinami, the player meets up with Theo again when they both go to Nuclear Plant Omicron to help the Rangers defend it against a potential attack. The player and Theo team up briefly to battle a pair of Nuclear Pokémon on Omicron's western shore. Once the Nuclear Pokémon on the island are defeated, Theo rejoins the player as CURIE and Urayne emerge from the power plant. When Cameron suddenly shows up on his Yatagaryu (revealing that he not only survived the blast at Epsilon but also that he has been following CURIE since his disappearance), Theo is both stunned and relieved to learn that his father is still alive. He rushes to Cameron's defense when CURIE threatens him; Cameron, fearing for his son's life, quickly jumps between Theo and Urayne, taking the attack meant for Theo and lapsing into a coma.
Although worried about his father, Theo volunteers to join the player on the mission into the Hazard Zone to apprehend CURIE and Urayne, where he is responsible for helping make sure the player gets safely to Nuclear Plant Zeta. He teams up with the player at various points within the Hazard Zone to help defeat hordes of Nuclear Pokémon, although he is initially focused more on trying to catch an unusual Pokémon he noticed as they got closer to Zeta. After catching a Hazma, he teams up with the player for the final stretch of the Hazard Zone before arriving at Zeta. Although Kellyn orders him not to go into Zeta (an order that Theo argues), Theo winds up inside anyway when Urayne suddenly appears and attacks both him and the player. He is then taken hostage by CURIE and trapped inside a stasis tank until the player defeats CURIE and rescues him.
After returning to Ranger HQ after the Zeta mission, Theo is reunited with his father, who awoke from his coma while they were gone. He remains with Cameron in Bealbeach until taking on the Championship challenge. Both he and the player rise quickly through the ranks, and are the two finalists for the title of Tandor Champion; however, their battle is interrupted first by the appearance of Nuclear Actan, then by CURIE. Theo helps evacuate the arena while the player battles CURIE. Afterward, he forfeits what would have been the final championship match, allowing the player to be named the Tandor Champion.
Post-game, Theo only stays in Moki Town briefly before leaving; he vows to continue training his Pokémon and re-challenge the Championship once more, where he hopes to have his "fated rematch" with the player.
At the start of the game, Theo is very immature, and tends to cry when things don't go his way, such as if he loses when receiving the starter weak against the player's. Nevertheless, he is a skilled Pokémon trainer who connects well with his Pokémon. He is always one step behind the player and expresses constant frustration over that fact. His greatest idol is his father Cameron, and he aims to someday be a skilled trainer just like him.
Theo starts to mature later in the game. Although still somewhat bold and reckless, his experiences have made him calmer and more in control of his emotions. His interactions with the player start to change as well; he starts to see both himself and the player as strong Trainers in their own right, both growing stronger together from their experiences rather than one having to be better than the other. By the end of the game, he is ready to make his own path in life rather than always copying the player.
Theo receives a starter with a type disadvantage to the hero's. If he loses, he believes it has made him unable to win battles against the hero. When starting his journey. his dad Cameron captures a few Pokémon for him to use, which he brings out in his first battle against the player in Nowtoch City (despite only having a single Pokémon added to his team). His team will vary depending on what starter the player chose.
Nowtoch City Battle
This battle occurs after the player has defeated Maria.