Obviously, Harry Potter himself. There could also be Japanese wizard characters who are his opponents in the dueling tournament. Maybe Hermione and Ron could come along with Harry, or they could be back at Hogwarts while Harry is in Japan, but still communicate with him. There might also be some Japanese magical teachers or judges in the tournament.
Harry is the main character for sure. Then, there could be some original Japanese characters created specifically for this fanfic. They might be young wizards training hard for the tournament, or experienced wizards who are mentoring the participants. Additionally, some of the characters from the original Harry Potter series like Hagrid could make a cameo appearance, perhaps sending Harry some magical item from Hogwarts that helps him in the tournament.
The main character would be Harry. There could be a Japanese wizard who is like a prodigy in dueling and becomes Harry's main rival. There may also be some side characters like a Japanese witch who helps Harry understand the local magical customs. And don't forget about the tournament organizers, who could be important characters in the story, setting the rules and creating the challenges for the dueling tournament.
If you want the clearest chapter reference, go to Chapter 125. Early on, Next, it was Qi Tianyu's turn.; later, the chapter leans on Although he didn't know what Jiang Li had chosen, just from her recent....
If you want the clearest chapter reference, go to Chapter 111. Early on, The main room's door was pushed open just a crack, and the head of the...; later, the chapter leans on Miss Jiang, you're still up, that's great..
Chapter 98 is the strongest chapter-level answer because it starts from Jiang Li listened to the end and didn't have much of a reaction, only... and follows through on That's why she's not scared.]] Everyone held their breath, clearly, the....
A practical answer is Chapter 112, because it turns the idea into a concrete scene. The chapter opens with These five people are supposed to be at odds., adds pressure through Do you remember that bowl of Mushroom Soup?" Jiang Li smiled, "Your..., and ends on [Hahahaha why is the Princess starting.
A practical answer is Chapter 95, because it turns the idea into a concrete scene. The chapter opens with Hearing others praise her daughter, Aunt Lin was smiling so wide she..., adds pressure through And once everything is settled, escaping might become very difficult for..., and ends on Your.
The best chapter pointer here is Chapter 138, "From now on, let's get to know each other again". It opens with Is there such a good deal? and closes on Could it be that the poison also had a side effect of making a person more....
A practical answer is Chapter 152, because it turns the idea into a concrete scene. The chapter opens with While most of the villagers of Chixia Village were gathered at the village..., adds pressure through Is every man in the world supposed to be hers?], and ends on That shouldn't be." Zhao.
The best chapter pointer here is Chapter 109, "Not One Deserves to Stay on His Friends List!". It opens with And another thing," Jiang Li said, taking out a tissue to wipe her lips,... and closes on [You're indeed more likable than me, the type of likable where people....
Chapter 120 is the strongest chapter-level answer because it starts from Facing the sudden and somewhat strange request from Jiang Li, Director Sun... and follows through on Drink this." Wen Jiaojiao looked at the half cup of thick substance:....
Chapter 134 is the strongest chapter-level answer because it starts from After a brief noon rest, the production team was up to their antics again. and follows through on [Are you kidding me, is Jiang Li putting on an act again?.