Well, Sandor Clegane, the Hound, could be a popular character to interact with Ash. His rough exterior and complex personality would be a contrast to Ash. Also, Samwell Tarly. His love for knowledge and his kind heart could make him a great friend or ally to Ash. And Melisandre. Her connection to the Red God and her prophetic abilities would create some really intense and mysterious interactions with Ash.
One interesting plotline could be Ash having to adapt to the unique political system in Westeros. Maybe Ash gets involved in the power struggles between the great houses. Another could be Ash forming unexpected alliances, like with the wildlings. And Ash might also bring some new skills or knowledge from their original world that gives them an edge in Westeros.
Definitely his adoptive father, Giovanni. Also, the members of Team Rocket like Jessie and James would probably have a different relationship with Ash. They might see him as a sort of prodigy or future leader. And his original friends like Misty and Brock might cross paths with him, which would lead to some very tense and interesting interactions.
Jessie and James are the obvious ones. They are the main members of Team Rocket. Jessie could be constantly trying to outdo Ash in Pokémon battles or contests of skill. James, on the other hand, might be more conflicted about having Ash around. He could secretly admire Ash's kindness towards Pokémon and start to have second thoughts about their Team Rocket activities.
The OC themselves are often popular characters. Since they are original creations, they can have all sorts of unique personalities, abilities, and backstories. For example, an OC could be a shy but brilliant Pokemon trainer who gradually overcomes their insecurities with Ash's help.
In the fanfiction, the fan girls may interact with Ash in a more collaborative way. They could join him on his travels for a short time. During this period, they help him with tasks like cooking for the pokemon or setting up camp. They also exchange stories about their experiences with pokemon, and the fan girls might learn a thing or two from Ash about being a better trainer.
Ash is often very enthusiastic and adventurous. Macy might be more cautious. So their interactions could start with Ash suggesting a daring plan, like exploring an old, abandoned Pokemon gym, and Macy being hesitant at first. But Ash's confidence might convince her to join in. Or Ash could be showing off his Pokemon skills to Macy, and she would be impressed and eager to learn from him.
Ash, being the enthusiastic and determined trainer he is, would likely approach Roxie with great respect for her as a gym leader. He would probably be eager to learn from her about different Pokemon types and battle strategies. Roxie, on the other hand, might be initially a bit reserved but would gradually warm up to Ash's energy.
Well, often the female characters in these fanfictions are very protective of Ash. For instance, Misty might get jealous if another girl gets too close to Ash. They also support Ash in his Pokemon battles. Like May could give him some useful advice on contests that can also be applied to battles sometimes.
He could interact with Serena again. Since they have a history in Kalos, they might have new adventures together. Also, Clemont and Bonnie are likely characters he would meet up with. They could continue their friendship and maybe work on some new inventions together with Clemont, or have fun times with Bonnie and her love for Pokémon.
He might interact with Jon Snow. They both have a sense of honor and duty, so they could become friends or allies. Maybe they would fight together against the White Walkers.