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.
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.
A practical answer is Chapter 102, because it turns the idea into a concrete scene. The chapter opens with When the camera cut back, the three of them had already returned to the..., adds pressure through Sadly, his forehead was injured, wrapped tightly in bandages, so she..., and ends on The.
Chapter 138 is the strongest chapter-level answer because it starts from Is there such a good deal? and follows through on Could it be that the poison also had a side effect of making a person more....
A practical answer is Chapter 188, because it turns the idea into a concrete scene. The chapter opens with With a "snap," the pencil Jiang Cheng was using to draw broke on the paper., adds pressure through Not to mention their utterly chaotic fertilizer spreading actions later., and ends on I.
A practical answer is Chapter 141, because it turns the idea into a concrete scene. The chapter opens with It had happened in the afternoon when Aunt Ying's son and daughter heard..., adds pressure through Director Sun has already agreed, and the audience in front of the screens..., and ends on.
A practical answer is Chapter 164, because it turns the idea into a concrete scene. The chapter opens with Damn, the production team is so shady, they're still doing a live..., adds pressure through Let's congratulate Jiang Yan for getting— a one-point card!" Jiang Yan: ?, and ends on The current.
A practical answer is Chapter 120, because it turns the idea into a concrete scene. The chapter opens with Facing the sudden and somewhat strange request from Jiang Li, Director Sun..., adds pressure through Netizen: Wen Jiaojiao had been acting for years without much fame, but now..., and ends on.
The best chapter pointer here is Chapter 121, "Jiang Li Gains 5 Points for His Meritorious Rescue!". It opens with Seeing that Wen Jiaojiao's condition had improved significantly, Director... and closes on You're supposed to be a man who conquers mountains!] Experienced and....
The best chapter pointer here is Chapter 176, "Su Yinwan: I Have a Stronger Taste". It opens with Shang Shaojing didn't suspect anything and popped the greens into his... and closes on Everyone intentionally avoided that plate of ugly-looking but darkly....
A practical answer is Chapter 158, because it turns the idea into a concrete scene. The chapter opens with Reluctantly, Jiang Yan had no choice but to forcibly join the group for..., adds pressure through Don't make me laugh, Jiang Li looks better even without makeup than she..., and ends on we're.
A practical answer is Chapter 138, because it turns the idea into a concrete scene. The chapter opens with Is there such a good deal?, adds pressure through From now on, let's get to know each other again." "I'm Wen Jiaojiao,..., and ends on Could it be that the poison also had a side effect of.