Obviously, Harry's parents, James and Lily, would be main characters as their portraits are central to the story. Dumbledore could also be an important character. He might be the one who either hid the portraits or knew where they were all along and guides Harry to them.
In addition to Harry, his parents are the main ones. However, Hermione and Ron could be important characters too. They could be with Harry when he discovers the portraits. Hermione might start researching about the magic used in the portraits right away, while Ron could offer his support and his own insights based on his family's knowledge of Hogwarts and its secrets. There could also be a ghost, like Nearly Headless Nick, who has some knowledge about the portraits and helps Harry understand their significance.
In this fanfic, perhaps Harry stumbles upon the portraits while exploring a hidden part of Hogwarts. He might have an emotional moment as he sees his parents for the first time in this way, and the portraits could start interacting with him, sharing long - lost memories and advice.
Well, apart from Harry, there might be an older, experienced sailor. This character has been in the Royal Navy for years and has a wealth of stories and knowledge to share with Harry. Then there could be a group of fellow trainees who are all going through the same experiences as Harry. They support each other, but also compete in a friendly way. And there could be a mysterious figure on the ship, perhaps a stowaway or a spy from another country, which adds an element of mystery and danger to the story.
The portraits could offer him emotional support. Just seeing his parents' faces might give Harry the strength to face difficulties. For example, when he's feeling down or facing a tough decision, looking at the portraits could be like getting a warm hug from his parents, even if they're not physically there.
Hermione could follow Harry to Japan to continue their magical adventures together. She would be eager to learn about Japanese magic and study in their libraries. Another character could be a Japanese wizard who becomes Harry's guide, showing him around the magical places in Japan and teaching him about local magic.
He would probably be overjoyed. Harry has always longed for a connection with his parents. Getting their portraits would be like getting a part of them back. He might just stand there, staring at the portraits with a big smile on his face, unable to take his eyes off them.
Obviously, Harry Potter is the main character as he is the one trying to save his parents. His parents, James and Lily Potter, would also be important characters even though they are the ones being saved. There might also be other characters like Dumbledore. Dumbledore could play a role in guiding Harry in his mission, either by providing advice or by giving him some magical tool or knowledge that helps him in his quest to save his parents.
Dumbledore could play an important role too. Even though he's passed away in the original series, in fanfiction, his spirit or his past actions could be related to Harry's inheritance. He might have left some clues or instructions for Harry to find out about his true inheritance.
There could be the Dragonborn. If Harry is in Skyrim, he might cross paths with the Dragonborn, and they could either become friends or rivals depending on the situation.
Hermione would definitely be one of the main characters. She would be very curious about Death's presence at school. She might try to research about it in the library, and her inquisitive nature would lead to some interesting interactions with Death. For example, she could ask Death about the secrets of the afterlife or some ancient spells related to death magic.
There might also be some new or minor characters from the wizarding world that are introduced specifically for this fanfiction. For instance, an old healer who has a unique method of restoring eyesight that no one else knows about. This character could play a crucial role in Harry being able to see again.