Their relationship could start with mutual respect for each other's skills with Pokemon. Ash is known for his battling prowess and Gardenia for her knowledge of Grass - type Pokemon. They might begin by exchanging tips on training and caring for Pokemon. Then, as they spend more time together, they could develop a friendship based on shared adventures. For example, going on a quest to find a rare herb that can heal sick Pokemon.
They could develop their relationship by sharing their dreams. Ash could talk about his goal of becoming a Pokemon Master, and Gardenia could share her dreams of making her gym the best in the region. By supporting each other's dreams, they form a stronger bond. For instance, Ash could help promote Gardenia's gym while Gardenia could give Ash some special training for his Pokemon.
Perhaps a plot where Gardenia has a special Pokemon that is sick, and Ash, with his experience in traveling and helping Pokemon, offers to help. They go on a quest to find a rare herb that can cure the Pokemon. Along the journey, they encounter other trainers who are also in need of help, and they have to decide whether to focus on their own mission or help others first.
Perhaps there's a plot where Gardenia challenges Ash to a friendly battle. But during the battle, an unexpected event occurs like a sudden storm or the appearance of a wild, powerful Pokemon. They then have to put their battle on hold and cooperate to deal with the situation. Afterward, they realize that they have a lot more in common than they thought and start to explore a romantic relationship.
Sometimes, other trainers or characters from the Pokemon world might make appearances in the fanfiction. They could be friends, rivals, or mentors to Roark and Gardenia. For example, another trainer might challenge them to a double battle, or an older and more experienced trainer could give them advice on how to better train their pokemon.
Their relationship could develop through mutual respect for their love of Pokemon. For example, James might show Gardenia some unique ways he's found to care for Pokemon during his travels, and Gardenia could teach him more about the nuances of grass - type Pokemon.
In some fanfictions, there could be a story about their friendship development. Maybe they start off as rivals, with Roark being more focused on the strength of his pokemon and Gardenia on the unique abilities of hers. But as they interact more, they realize they can learn from each other. They could exchange training techniques and even help each other in pokemon battles against common enemies.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
One plot idea could be that James and Gardenia team up to save a rare Pokemon from a poacher. They start off not getting along but gradually learn to respect each other's skills.
Ash could be a Pokémon trainer with a different starting point in this fanfiction. Born in 1987, by 1997 he might have already made a name for himself in local Pokémon battles. He could have a team of Pokémon that he rescued from difficult situations, like a Charmander that was abandoned by its trainer. His story could revolve around giving these Pokémon a second chance and proving that Pokémon don't need to be 'perfect' to be strong.