The main character could be a lesbian who has moved to a different country. In the new country, she initially had no friends or support system. However, she starts to attend local LGBT events and gradually builds a network of friends and maybe even finds love, thus no longer feeling alone.
It could be a young lesbian just coming to terms with her sexuality. She might have been struggling in a conservative family or school environment where she felt like an outcast, but then she found a mentor or an older lesbian who guided her and made her feel that she was not alone.
Perhaps it's the story of a lesbian who overcame personal insecurities or family rejection that made her feel alone. She might have started a journey of self - acceptance and along the way met others like her, which changed her situation from being alone to being part of a supportive group.
It could be a young adventurer who decides to explore an abandoned house at night.
The main character could be a mischievous little boy. He's always getting into small troubles around the house when his family is there. So when he's home alone, he really has a field day. He might try to build a huge fort in the living room or attempt to cook something really elaborate but ends up making a big mess.
Perhaps the main character is a boy named Sam. Sam is a bit of a troublemaker usually, but being home alone changes him. He realizes the importance of responsibility. He has to take care of his pet dog while his parents are away. And when some unexpected events happen, like a small fire in the kitchen because he tried to cook something fancy, he learns from his mistakes and becomes more mature in the process of dealing with these situations.
Since I haven't read this specific fanfic, it's hard to say for sure. But it could be Harry Potter, of course. Maybe Hermione and Ron are also involved in helping the 'no longer alone' aspect, like they do in the original books, by being his friends and comrades.
A war veteran could be the main character. They might have come back from the war with PTSD and find it difficult to connect with people. At a community event, the crowded room full of happy and carefree people makes them feel even more isolated as they struggle with their own inner demons.
It could be a new character, an orphan who has just discovered they are a wizard. They come into the wizarding world all alone, not knowing anyone or anything about this new place. They stand alone because they don't have a family name or background to rely on like the Weasleys or the Malfoys.
It could be a mermaid or merman who is alone in the ocean for some reason, maybe banished from their community.
The main character is likely Ash, as it's in the title 'ash trains alone fanfic'.
"The Strongest Heavenly Doctor,""Dance of the Fingers,""My Body Can Be Digitized," and "Protagonist Trainer" were all novels about the protagonist walking over alone. Among them,"The Strongest Heavenly Doctor" was a novel about urban life, with the protagonist making a name for himself in the big city."Dance of Fingers" was an eSports novel, with the protagonist running away from home because of eSports and chasing his dreams in a distant city."My Body Can Be Digitized" was a novel about time and space travel, with the protagonist crossing different worlds to survive."The Protagonist Trainer" was an oriental fantasy novel, with the protagonist being a legendary figure who walked in various worlds. I hope you like this fairy's recommendation. Muah ~😗