Since I don't have access to the 'an old friend short story', I can't say for certain. But usually in such a story, the main characters would be the old friends themselves. There could also be secondary characters like family members or mutual friends who play a role in their relationship, either supporting or causing some sort of friction between the two main friends.
In the 'false friend short story', the main character is often the victim of the false friend's actions. This character is usually trusting and naive at the start. The false friend is the key antagonist. They may seem friendly and charming on the surface but are hiding their true selves. Additionally, there may be a sidekick or confidant of the main character who might notice something strange about the false friend earlier on and try to warn the main character, adding an extra layer of drama to the story.
Since I don't know the specific story, it could be two friends, perhaps named John and Tom. John could be the one in need and Tom could be the helper.
Since we have no information about the 'a little friend short story' other than its title, we can only make assumptions. The main character is likely to be the 'little friend'. This could be a young child, perhaps a boy or a girl. There may be other characters like a wise old man or woman in the neighborhood who gives advice to the little friend. Or there could be a group of children who are friends with the main character and they all go on adventures together, sharing joys and sorrows.
I would assume that the main characters are the two friends. One is the protagonist, the person whose perspective we are seeing the story from, and the other is the friend who is no longer in their life as closely. There might also be some secondary characters who play a role in the events that led to the friendship changing, like other mutual friends or family members who influenced the situation.
There's a good chance the main characters are two old friends. They could be characters who have known each other for a long time and their relationship is based on shared memories and experiences that are filled with joy. Maybe they meet again after a long time apart and their interaction forms the core of the story.
There might be a group of kids who grew up in the old neighborhood. They could be the main characters as they experience the joys and sorrows of living there. They play in the streets, have adventures in the local parks, and form friendships that last a lifetime. And as they grow up, they see the neighborhood change around them.
There's the boy, who is the central human character in 'old yeller short story'. He is the one who forms a strong bond with Old Yeller. Old Yeller, the dog, is the other main character. He is brave, loyal and loving. Then there are the boy's parents and siblings who also play a part in the story, but it's the boy and Old Yeller's relationship that drives the plot forward.
I'm not sure as the title doesn't tell us directly, but it could be the former owners of the old house.
The main character is Sultan, the ageing dog. His owner is an important character too. There could be other characters such as the neighbours who might be mentioned in relation to the events on the farm. Also, if there are any threats like wild animals or thieves, they can be considered as characters in a sense as they drive the plot forward and Sultan's actions are in response to them.
Often in such stories, there could be an old recluse who lives near the old mansion. And maybe a young journalist who is trying to write a story about the mansion's history. These two could be main characters, with the recluse having a lot of knowledge about the place and the journalist trying to uncover the truth from him.