Forrest debuts alongside Brock and his family in Showdown in Pewter City! where he was seen with the other sibilings tried to stop Ash from hurting his Onix even further during the rematch of the gym battle between him and Ash who wants to gain the first gym badge due to the water sprinklers afflicted on it. Although, Ash stops the match due to their battle is unfairly as he leaves the Gym with Pikachu who is exhausted as Brock tells him that their battle isn't over yet. But Brock follows Ash after knowing he was defeated by him, he gives him the Boulder Badge. Brock's siblings are mentioned by Brock when he gives his father Flint some errands for their family before travelling with Ash and Misty.
When Brock returns to his hometown after his journey in Johto in A Family That Battles Together Stays Together!, Forrest was happy to see his older brother's return alongside his father Flint who warns Brock not to come back due to his wife Lola redecorating the Pewter Gym into an aquatic gym. Forrest also watches his father using his Golem to fight against his mother who uses Marill and realizing that rock hates water leading to his mother Lola beats his father Flint. Forrest also likes Brock's caring to his Pokemon due to his wish of becoming a Pokemon Breeder. When their father decides to make the gym into a two-type Pokemon, Forrest angrily mad at his decision to make the gym into a laughing stock and he runs away. Brock was told by his younger sister that Forrest wanted to become the next Gym Leader much to his surprise.
That night, Forrest was later convinced by Brock about his dream to become the Gym Leader and knowing that he can do it and also tells him that rock Pokemon will win another day. In the next day, Forrest was later amazed to see Brock fought their mother for the gym using Onix against her Mantine who manages to defeat it and finally reclaimed its former glory and was happy to see the rock-type beats a water-type. After his father Flint failed to uphold the ideals of the Pewter Gym, Brock left Forrest in charge of the gym as the Gym Leader and he happily lent his Onix to him which is a surprise to him and he tells him that Onix is well-trained much to his happiness. Forrest and his family bid farewell to Brock who is now travelling to Hoenn to meet up with Ash, May and Max.
In You Never Can Tailow! Brock saves Ash, May and Max from a flock of Tailow using his Forretress. Brock also tells them that he left Geodude, Onix and Crobat from his home in Pewter City so that Forrest can practice in how to handle his Pokemon and his skills in breeding.
When Brock later returned in the episode Grating Spaces!, he and his siblings reunites with his older brother who informs him about the gym has been remodeled for the second time. Forrest let his older brother Brock, Ash and May to show them the inside of the gym and he found out that his Onix had evolved into a Steelix under Forrest's care who carries Brock on purpose as a reunion. Brock was very proud to see Forrest to his care of his Pokemon. He also found the gym painted in horrible colors for the second time and it was done by the gym remodellers which is Jessie and James in disguise.
Forrest became the judge of the battle between Ash and Brock against a disguised Jessie and James who uses both Butch and Cassidy's Aggron and Charizard to fight their Pokemons Steelix and Donphan. When Max informs them that Meowth from Team Rocket is stealing the Pewter Gym's Poke Balls and both Jessie and James' disguised are now blown by two Pokemons to finally reveal themselves. Forrest becomes mad when Meowth was lying that he didn't steal it but he recognizes his face and he releases Geodude to use Tackle on Meowth which he reveals their motives of stealing. Both Brock and Forrest are mad for what they have done as Brock calls Team Rocket a disgrace. Forrest also became furious to see what they done for trying to steal the Pokemon and was unforgivable and angrily challenged them much to Brock's surprise. Forrest angrily let Geodude to use Sandstorm on Team Rocket. Altogether, Steelix and Geodude uses both a Iron Tail and a Mega Punch towards both Pokemon to knock them down until they are blasted off by Pikachu's Thunderbolt followed by Donphan's Hyper Beam, Steelix's Dragon Breath and Geodude's Sandstorm. Brock was impressed to see Forrest in action which he became happy to his older brother's compliment. Forrest and his family later farewell to Brock, Ash, May and Max who are now travel to find the Battle Pyramid which is manned by Pyramid King Brandon to get the last and final symbol.
At the time of Brock's journey in Sinnoh, his father Flint gives his Rhydon to Forrest which is later evolved into a Rhyperior in his care. When Brock later returned from Sinnoh while he currently studies as a Pokemon Doctor, he discovered that Forrest's right to be the Pewter Gym Leader was under contest. Forrest fights Nurse Joy from the PIA using his Rhyperior while being supported by his family and Brock's Pokemon. After Forrest fought a well fought battle with his Rhyperior against Nurse Joy's Latias, he lost the fight but was given the official title of Gym Leader due to his battle experience and Nurse Joy allows them to operate the Pewter Gym.