Treecko first appeared in It's Treecko Time! where he attacked the group while they were eating lunch and stole their food. The group later witnessed Treecko giving the stolen food to a group of Zigzagoon. Parker explained that Treecko would often steal food from Trainers passing through the forest and give it to the Pokémon who weren't able to get it themselves. Lewis expressed his desire to capture Treecko, but thought better of it when he realised that the Pokémon in the forest needed him.
Later, Lewis and Treecko got separated from the others when Kai attacked them in an attempt to steal the forest Pokémon. When Kai caught up with them, Treecko and Lewis teamed up to battle him. During the battle, Kai's machine began to self-destruct. In order to protect Treecko from the explosion, Lewis threw a Poké Ball at him just as the machine exploded. After the explosion, Lewis discovered that he had caught Treecko. He attempted to release Treecko, but he refused and told Lewis he wanted to travel with him. Treecko left the food-gathering duties to a Nuzleaf and left the forest with our heroes.
In Geodude Rocks the House! Lewis used Treecko as his second Pokémon during his first Gym battle against Roxanne. Despite having the type-advantage, Treecko was knocked out by Geodude's Rollout.
Treecko was used last in Lewis' rematch against Roxanne in A Rock Solid Rematch! It battled against Roxanne's Nosepass and was almost defeated by the Rock-type. However, by using a combination of Absorb and its newly-learnt Energy Ball, Treecko was able to defeat Nosepass and win Lewis the Stone Badge.
Treecko has been shown to be kind and compassionate from its debut, where it stole food from Trainers to give to the less-able Pokémon living in the forest. Due to this, Treecko can be described as a Pokémon Robin Hood, a parallel that is further highlighted in Treecko's preference to wear a Robin Hood-style hat. Treecko is also shown to be a strong battler, managing to defeat strong opponents such as Roxanne's Nosepass.