Marlon appears in Pokémon Black 2 and White 2 as the eighth and final Gym Leader that the player challenges. He is a big-hearted Pokémon Trainer who gives passionate advice. Marlon also seems to be a very relaxed and carefree man, as shown by his obliviousness as to who Team Plasma are and what they do. He asks the player if Team Plasma are bad people.
Marlon specializes in Water-type Pokémon. Upon defeating him, he will reward the player with a Wave Badge and TM55 (Scald). Outside of battle, Marlon is a fisherman who catches Water-type Pokémon which he would later use in future Gym battles.
Upon arriving in Humilau City, Marlon is absent from the Gym, and the player is sent to find him. The player meets Marlon just as they head north of the Gym, in which he returns to it and awaits a Gym battle. After the Gym battle, Marlon jumps into the ocean and swims away. Upon exiting the Gym, Hugh walks up and congratulates the player on gaining all eight of Unova's Gym Badges, and then their attention turns to Team Plasma, during which Marlon walks up and asks about them.
He heads off to the Seaside Cave to find the Plasma Frigate. Later on, at Route 21, he is seen jumping off the Plasma Frigate after lowering the boarding plank, and then heads back to the Gym, leaving the player and Hugh to deal with Team Plasma.
Jingle: "Woo hoo! All right! It's 'The National Gymquirer'!"
Page: "That's right! We collect the hottest gossip about Gym Leaders seen around town! Here is today's letter. From someone going by the name MisterFisher! Thank yooou! 'Marlon was swimming the other day, and he just kept going faster until he shot out of the water and into the sky! That swimming style could completely dominate the world of swimming.' Hm... That is amazing, but nobody else can do that. Actually, can you even call jumping into the sky swimming? 'The National Gymquirer'! We eagerly await your letters!"
Marlon is a spirited Gym Leader with a laidback personality. He has a strong team of Water Pokémon, which perform well in the Humilau Gym's open ocean battlefield. Marlon utilizes offense to bring down his challengers and takes advantage of the Gym's aquatic setting.
Mantine is Marlon's ace Pokémon. It was the second Pokémon that Marlon used against Cameron in their Gym battle. It quickly knocked out Ferrothorn with a combination of Wing Attack, Bullet Seed, and Psybeam. Afterwards, Mantine faced off against Cameron's female Samurott and was knocked out without landing a single attack on her.
Marlon debuted in Hallway Hijinks, where he participated as a competitor in the Pokémon League. After battling his way through Victory Road, he eventually made it into the top 8. In The Tournament Continues, Marlon faced Hood Man in the quarterfinals. Despite managing to disable the moves of Hood Man's Beheeyem, Marlon ended up losing, allowing Hood Man to advance to the next round.
In Something Suspicious, Marlon attempted to head back home, only to be approached by White. When asked on what he knew about Hood Man, Marlon revealed his opponent had managed to somehow enter the League without collecting a single Gym Badge. White decided to expose Hood Man for cheating, but was stopped by Marlon, who was perfectly fine with losing since his opponent was a strong Trainer. Marlon stated that he found Cheren more suspicious and suggested that White begin investigating him next.
Jellicent is Marlon's only known Pokémon. He has been with Marlon ever since he was a Frillish. Marlon used him to battle Hood Man's Beheeyem at the Unova Pokémon League. Although he put up a good fight, Beheeyem defeated him with Shadow Ball, giving Hood Man the victory. Later, he was healed by White's Alomomola, Nancy, in an attempt to get Marlon's trust.