Obviously, Ash is one of the main characters. Since it's about him being a legendary Pokemon in this fanfiction, he's central to the story.
There could also be some legendary Pokemon as main characters. For example, Mewtwo might be involved. Mewtwo is a very powerful and intelligent Pokemon. It could either be a mentor to Ash in his journey as a 'legendary' or a rival who challenges Ash's newfound power.
Pikachu is likely to be a main character as well. Pikachu has been Ash's loyal partner throughout his journey, and in this fanfiction, Pikachu might have a special role in helping Ash understand and control his new legendary status.
Obviously, Ash is one of the main characters. He's the protagonist who has a passion for pokemon and is always eager to become a pokemon master. And the legendary pokemon he encounters or befriends would also be important characters. For instance, if it's Lugia, Lugia would play a major role in the story as it's a very powerful and mysterious pokemon.
Obviously, Ash is one of the main characters. He is the trainer who gets the legendary starter. And of course, the legendary starter itself is also a main character. It has its own personality and abilities that play a big role in the story.
The villains in the fanfiction can also be considered main characters in a sense. They might be after the power of the legendary Pokemon. There could be a team of bad guys who are trying to capture and control a Groudon or a Kyogre to cause chaos in the Pokemon world, and Ash has to stop them. Their actions and plans create the conflict that Ash has to overcome in the story.
Obviously, the Legendary Pokemon would be a main character. It could be a Pokemon like Mewtwo, who is often depicted as having a complex personality. It might start off as a mysterious and powerful entity that Ash has to befriend or battle against.
Articuno, Zapdos, and Moltres are quite common. Their elemental powers and the mystery surrounding them make for great fanfiction material. Ash might encounter them during his travels and have to either help them or face off against them in some way.
One of the Legendary Pokemon could be Giratina. Despite its somewhat dark and otherworldly nature, Giratina might be intrigued by Ash's light - heartedness and his ability to make friends with all kinds of Pokemon. Dialga and Palkia could also be part of the harem. Dialga, with its control over time, might see Ash as an important factor in the future of the Pokemon world. Palkia, in charge of space, could be interested in how Ash navigates different regions and spaces in his journey. And don't forget about Ho - Oh. Ho - Oh is a symbol of hope and might see Ash as a bringer of hope in the Pokemon world, thus joining his harem.
Arceus is quite popular. Given that it's considered the creator of the Pokemon universe in the official lore, it often has a significant role in fanfictions. Mewtwo is also very common. Its powerful psychic abilities and complex backstory make it a favorite for fanfic writers to explore. Another popular one is Lugia. With its majestic appearance and its connection to the sea, it's often a central figure in many stories.
There are also fanfictions where Ash catches Rayquaza. This could be set during a time when the sky is in chaos due to some dark energy. Rayquaza is drawn to Ash because of his pure heart and determination. Ash, with his usual courage, teams up with his pokemon to assist Rayquaza in restoring balance. As a result, Rayquaza allows Ash to catch it.
Since it's an 'ash the pokemon fanfiction', Ash is the central figure. But it could also feature his friends from the original series like Misty and Brock. And of course, the villains he encounters would be important characters too. There may be new characters created by the fanfiction writer, such as a mysterious trainer who has a rare and powerful Pokemon that Ash has to face.
The appeal lies in seeing Ash in new and exciting situations. With legendaries involved, his journey becomes more epic. It's like taking the underdog story of Ash in the anime and supercharging it.