I'm not sure specifically as it's a fanfic. But it could be characters from the original source material that the fanfic is based on, who are now put in situations where they fight alone.
It could be about a story where characters face challenges alone in a fight. Maybe it's a fan - created story set in a particular fictional universe where the hero has to battle enemies without much help.
I'm not sure specifically as I haven't read the novel in detail. But they could be scientists who are on the quest to find other life forms. Maybe there's a lead scientist as a main character who drives the story forward with their research and discoveries.
Since I haven't read the 'alone on the water fanfic', I can't say for sure who the main characters are. But it could be a lone sailor, an adventurer, or someone who has been stranded on the water.
It could be a young trainer who prefers to journey alone. They might be the central character, facing Pokemon battles and exploring the world on their own.
It's possible that there are ordinary people who get caught up in the events related to the discovery of alien life. Maybe a journalist who is trying to report on the story, or a family that lives near a site where some strange alien - related activity is taking place. There could also be alien characters themselves, which would be interesting to see how they are portrayed in terms of their appearance, behavior, and intelligence.
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.
Don't forget villains. They are also important characters in this kind of fanfiction. There could be a villain who is jealous of the Avengers' strong family bonds and tries to break them apart. This villain could be someone new or an old enemy with a new vendetta related to the concept of family.
I'm not sure as there isn't much information given. But it could be an ordinary person who has to face challenges alone.
Well, usually it would include the main characters from the original 'After We Collided'. So, Tessa and Hardin would be the central figures most likely.
Since it's a RWBY fanfic, there are likely to be some of the main RWBY characters like Ruby, Weiss, Blake, and Yang. As for the knight, they would be a main character too. There might also be other characters from the RWBY universe that the knight interacts with, like Ozpin or Qrow.