The main character is Ash. But in some versions of the fanfiction, there could be new characters introduced that are related to the situation he's in. For instance, a hermit who lives in the forest where Ash is stranded. This hermit could either help Ash or be a hindrance, adding more depth and complexity to the story.
Gary could be a likely character. He has always been Ash's rival and there could be a plot where his jealousy or some other motive leads him to betray Ash. Another could be one of his so - called friends from the Pokemon League who might be influenced by power or some dark secret. Or, in some really dark fanfictions, it could be a Pokemon that Ash trusted deeply but for some reason turns on him.
Ash might meet some lone Pokémon trainers. These trainers could be on their own quests, similar to Ash's. They might have unique Pokémon or training methods that Ash can learn from.
There could be a lone traveler who Ash meets while he is alone. This traveler might have a unique way of training Pokemon that Ash has never seen before and could teach him some new tricks.
He might meet some local Sinnoh trainers who are experts in training specific types of Pokémon, like a trainer who specializes in ice - type Pokémon in the colder regions of Sinnoh.
There might be a young and inexperienced trainer who is lost. Ash, being the more experienced one, decides to help the trainer find their way. This new character could look up to Ash and in the process, Ash learns more about himself as a role model. It would be an interesting dynamic as Ash is usually the one being guided by his friends.
In 'ash is alone fanfiction', perhaps there are stories where Ash discovers new powers within himself during his time alone. He might have to rely on his wits and these newfound abilities to survive in a strange and dangerous situation, such as being stranded on an uncharted island filled with unknown Pokémon.
One interesting 'ash alone fanfiction' could be about Ash going on a solo adventure in an uncharted forest. He discovers new Pokémon species and has to rely solely on his own skills and the bond with his Pikachu to survive.
One common theme is self - discovery. When Ash is alone, he has to figure out who he is without his friends' influence. Another theme is independence. He has to rely solely on his own skills to catch Pokemon and overcome obstacles.
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.
I'm not sure as I haven't read this particular fanfiction. But since it has 'ash' in the title, it's likely that Ash is one of the main characters.