Well, it's hard to say it's a widely popular genre on its own. The Naruto fandom has a vast range of fanfic types. While the idea of Naruto being a cold and distant shinobi might attract some readers who like exploring different character portrayals, it's not as mainstream as some other common themes like Naruto achieving new powerful jutsus or his relationships with other main characters. There are so many different takes on Naruto in fanfics, and this rather 'edgy' or different view might only appeal to a niche group within the large Naruto fan base.
One reason could be to explore a different side of Naruto's character. In the original series, he's so warm - hearted, but by making him cold and distant, the writer can show how different events could have shaped him in an alternate way. Maybe he faced more hardships or betrayals that led to this change.
Another reason could be the influence of a powerful and dark entity. If Naruto is somehow possessed or influenced by a dark chakra or a malevolent spirit, it might change his personality completely. He could start to push away people who care about him because the entity wants him to isolate himself and be more vulnerable to its control.
Well, the shinobi rules in Naruto fanfic can be quite diverse. There's the rule of secrecy regarding jutsu. Shinobi are not supposed to freely share their unique and powerful jutsu with outsiders or enemies. Then there's the code of honor in combat. They should fight fairly, not use underhanded tactics like attacking an unarmed opponent. And of course, they are required to respect the hierarchy within the village's shinobi system.
Perhaps it's a shinobi who has retired after a long and eventful career. He could be someone who has seen the darker side of the shinobi world and wants to make sure Naruto doesn't go down the wrong path. He might have been a wandering shinobi who finally settles down with Naruto. For example, he could be a shinobi from a different village who has a unique set of skills to teach Naruto.
The author could show his coldness through his limited dialogue. For example, Harry might give short, curt responses to others. This would immediately signal his distance.
I'm not sure specifically what that particular 'the last shinobi naruto fanfic' is about as there could be many different interpretations and stories within fanfics. It might be about the last remaining shinobi in the Naruto universe, perhaps their final battles, or their efforts to preserve the shinobi way in a post - apocalyptic or changed Naruto world.
Well, in this kind of fanfic, usually there would be intense battles. Naruto is extremely powerful with his various jutsu like Rasengan. The Shinobi Alliance also has a lot of strong ninjas. Maybe the story would focus on Naruto having to face the combined forces of the alliance due to some misunderstanding. It could also explore Naruto's growth and how he tries to resolve the conflict without causing too much harm.
Well, it could be about a role - playing game set in the Naruto universe where players take on the roles of shinobi. Maybe it involves following the traditional Naruto storylines but with an RPG twist, like character leveling, quests, and battles in the style of a game.