Marlon was born and raised in Galar alongside his sister Nessa, but had a distaste in his home region due to the lack of water around him, having watched television shows about swimming and the sea since childhood. When he was still a young Trainer, he decided to go travelling and moved to Humilau City, Unova - famed for its aquatic land. He's a big-hearted Trainer who's famed for his passionate advice. He's also very relaxed and carefree, often keeping out of negative discussions and events. Despite this, he's shown to be extremely passionate for those he loves, and will do anything to ensure they're happy.
Marlon is a keen Surfer, often training in the open sea, coming into contact with random Trainers while Swimming. He heavily campaigned for the Humilau City Gym to become an officially recognized Gym, which ultimately led to his Gym replacing the Icirrus City Gym once Brycen stepped down. Marlon remained as the Gym Leader ever since, despite his ambitions to travel.
The Alexis Saga
Marlon first appeared in Best Wishes: Plasma Freeze, when Lexi came to battle against him. When he first met her, he was shown to have an immediate attraction to her, fumbling his words and complimenting her at many opportunities. The two battled for Lexi's final Unova Gym Battle in which Lexi won her final Badge. Immediately following their battle, Marlon asked if he could join her on her journey, which she agreed. He was a supportive hand to Lexi while she was battling and taking down Team Plasma, often protecting her and fighting in her honor.
When Lexi completed the battle against the UnovaElite Four, he and Lexi shared a moment, which seemed to suggest that his feelings towards Lexi were mutual, however, due to the following events, they couldn't act on.
In the events of M03 and M04, Marlon was an active role, causing a distraction from a group of hunters from capturing Lexi, ensuring her escape.
He didn't appear at all following this, although Hayley gave Lexi a message from him in New Beginnings saying that he'd come and see her soon.
Marlon reappeared in Fate Decided after being asked to visit Lexi in Kalos. Upon their re-meeting, he and Lexi once more shared a moment similar to the one Marlon previously had when he first met her, showing their feelings were still present. Elesa had mentioned that Marlon "hadn't stopped talking about her (Lexi)" since deciding to visit her. After helping Professor Sycamore and leaving Kalos, he kissed Lexi on the cheek before leaving.
Marlon returned to Kalos in X vs. Y: Destruction, after hearing the news about Lysandre's return. While there, he discovered that Calem had feelings for Lexi, and warned him off trying to start anything with her, resulting in a rivalry striking up between the two. He was then apart of the Kalos War, and was the first person to run in after Lexi when Parfum Palace was on fire, however was unsuccessful, being dragged out of the building after refusing to let her go.
At her funeral later in the movie, he and Calem make amends, and also mentions he plans to go 'traveling' following the funeral.
Xenial Destiny II
Marlon reappeared in Ruby & Sapphire alongside Shauna and Calem, after deciding to join the two in their journey to Hoenn. He seemed to have become close friends with both Calem and Shauna, claiming that they grew closer due to Lexi's death in a conversation with Roxanne. His team remained mostly the same other than the additions of Totodile and Corphish (the latter caught in Hoenn).
Like Calem and Shauna, he was oblivious to the fact Lexi was still alive, and upon discovering she was in fact alive and well, reacted much happier than his two friends did, and was happy to help with Team Aqua and Team Magma.
Marlon spent his time with the team helping them train along with training his two newest Pokémon. As their journey in Hoenn came to a close, he decided to return to Unova, but told Lexi he'd be back if ever she needed his help.
Marlon is first seen in the series when he runs into Lexi when she arrives at his sister's Gym. Marlon had been visiting family in Galar. In his time since last seeing Lexi, he reveals that the Humilau Gym was shut down and that he has been travelling ever since. When Lexi relays her run in with the mysterious Pokémon, Marlon asks to accompany her to aid her due to Sonia's research into the Darkest Day which may share a connection. She agrees, and Marlon once again joins Lexi on her journey. Before leaving Hulbury, Nessa gives him a Sobble to take care of while he travels. He later captures a Chewtle, which eventually evolves into a Drednaw.
Marlon's time with Lexi in the series opens up a deeper conversation within himself regarding his path in life. He often is shown very interested and intruiged with Sonia's musings with the history and lore of the region and often is the most proactive member of the team interms of searching for potential clues or aids to their discoveries. As Sonia pieces together the story behind Zacian and Zamazenta and the Darkest Day, Nessa and Marlon uncovered the secrets behind their family origins, discovering that their ancestors opposed the royal families decision to cover up the Pokémons history with Eternatus, instead replacing this with the first kings ancestor as the hero. They also discovered that they themselves were the creators of the Rusted Sword and Rusted Sheilds that were the sources of the Legendary Pokémon's powers.
He realises as time goes on he wishes to continue exploring in order to gather and retain data regarding Legendary and Mythical Pokémon, by remaining with Sonia for the time being before travelling once more once he found a research area he wanted to focus on.