Yes, there might be. The world of fanfiction is vast and diverse. With the popularity of both Harry Potter and the concept of Ryuji Jakkan (assuming it's a known or created concept), it's very likely that some fans have combined them in their creative works. However, it may not be as mainstream as some other Harry Potter fanfictions.
One common theme could be power and its consequences. Since Percy is an elder god, he has a great deal of power, and the fanfiction might explore how he uses or misuses it.
A common theme is friendship - turned - love. They start as friends and gradually develop deeper romantic feelings. Another theme is mutual support. They help each other through tough times, like Haru being there for Ryuji when he has family issues or Ryuji giving Haru confidence in social situations.
One common theme is friendship. Since Ryuji is part of the B Project, fanfics often focus on his friendships with the other members. Another theme could be his journey to achieve his dreams as an idol.
One common theme could be redemption. Given Kamoshida's actions in the game, fanfic might explore his path to redemption in relation to Ryuji. Another theme could be friendship or an unlikely bond forming between them.
Ninja Gaiden 3 Full Story is very challenging. The difficulty level ramps up as you progress through the game. There are complex enemy patterns to figure out and the bosses are no pushovers. You have to be precise with your attacks and dodges, and also manage your resources like health and weapons effectively.
One great Ninja Gaiden fanfic is 'The Ninja's Redemption'. It delves deep into the character's internal struggles while maintaining the action - packed essence of the game. Another one could be 'Ryu's New Quest' which takes the story in a new and exciting direction, exploring uncharted territories in the Ninja Gaiden universe. And 'The Hidden Blade' is also quite popular as it focuses on the mystery behind a powerful weapon in the game's lore.