Since the title focuses on Naruto not wanting to be alone, Naruto is the obvious main character. But it could also feature characters from his family, like his father Minato in flashbacks perhaps. Minato's presence, even if only in Naruto's memories, could be a source of comfort for Naruto and thus be an important part of the fanfic in relation to Naruto's feelings of loneliness.
Naruto would definitely be one of the main characters since it's in the title. And his friends like Sasuke and Sakura are likely to be important characters as well, as they are the ones who are often there for Naruto and can help him deal with his fear of being alone.
Well, it might be a fan - made story that delves into Naruto's internal thoughts regarding being alone. Since Naruto grew up without his parents and was often shunned by the villagers, he surely had many moments of feeling alone. This fanfic could be focusing on those deep - seated emotions and how they affect his actions and decisions throughout the story.
Iruka Sensei might be important too. He was one of the first people to show kindness to Naruto. He could be a key figure in making the village understand that Naruto is worth having back. Also, Sasuke might be involved. If he has reconciled with the village, he could see the value of Naruto's return and help in the process.
It could be a mermaid or merman who is alone in the ocean for some reason, maybe banished from their community.
There could be a mentor - like figure in the novel. This person might have some wisdom or experience related to the imperial system and tries to guide the main character. Maybe they understand her reasons for not wanting to be an empress and help her navigate the political and social implications of her decision. Additionally, there could be antagonists who want to force her into the role of empress for their own gain.
I don't know as I haven't read the light novel. But usually, there would be the girl who doesn't want to be the crown princess as the main character. There might also be a prince or some sort of royal figure who could be involved in the story, perhaps as a love interest or as someone trying to persuade her to take on the role.
There could be a group of characters. For example, a group of friends where one of them is the main focus, the one who really doesn't want to get hurt. The other friends may either understand and support this friend's fear or accidentally push them into situations that could potentially cause harm, leading to various conflicts and character developments within the group.
There could be some original Asian - inspired characters. For example, a young martial artist from a small village in China who Naruto takes under his wing. This character could be eager to learn from Naruto and would be one of the main focuses as he grows under Naruto's guidance.
The orphans themselves would be main characters. Each one could have their own backstory and personality. Some might be shy, while others could be more outgoing. They would be the ones directly affected by Naruto's care and teachings.
He might meet an old acquaintance who is also alone. They could share their stories and perhaps find some comfort in each other's company.
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.