Without having read it, it's hard to say for sure. However, it's possible that the main characters could include a soldier who is stationed near the gate and gets caught up in the events of the gaiden. There might be a local from the world on the other side of the gate who becomes a guide for the characters from our world. And perhaps there's a researcher type of character who is studying the gate and its properties in this side - story light novel.
The main characters include Sarada, who is the daughter of Sasuke and Sakura. Sasuke also plays a significant role as Sarada's father. And Sakura, Sarada's mother, is also an important character. They are the central figures around which the story of Naruto Gaiden anime mainly revolves.
I don't know for sure since we don't have more details about this 'the arena science fiction'. But typically, in an arena - based science - fiction, there could be a brave hero or heroine who is trying to survive or fight for a cause.
Maybe there are magical beings like wizards or witches. They would use their magic powers in the arena and in the forest. Also, forest creatures such as centaurs or sprites might be main characters as they have a connection to the fantasy forest.
I'm not entirely sure, but it's likely to feature characters related to the sword aspect in the 'Danmachi' world. Maybe there are sword - wielding heroes or characters who have a special connection to sword - making.
Boruto is an obvious choice. He and Sarada have a unique friendship in the Naruto universe, and their interactions can add a lot of depth to the story. They could go on missions together or have friendly competitions.
A good character to include in Ninja Gaiden crossover fanfiction is Bayonetta. Her over - the - top combat moves and witchcraft powers would be a great addition. Also, Ezio Auditore from Assassin's Creed. He and Ryu could exchange ideas about stealth and assassination techniques. And then there's Kirby. His ability to copy abilities could lead to some humorous and unique interactions with Ryu, especially if he copies Ryu's ninja moves.