It might be a character who was thought to be dead but actually survived. For example, someone from the Uchiha clan. He could be the last male and have a vendetta against those who wronged his clan. But as the story progresses, he learns the value of forgiveness and uses his power for the greater good of the ninja world.
Maybe the last male character is an outcast in the village. But as the story unfolds, he finds out that he has a special connection to an ancient prophecy. This connection makes him the key to saving the ninja world from a great darkness that is slowly spreading. He has to overcome his own insecurities and the distrust of his fellow villagers to fulfill his role in the prophecy.
Since I haven't read it, I can't say for sure. It could be an original character created specifically for this fanfiction, or it might be a male character from the Naruto series that has been given a new and different role as the 'last male nin'.
It could be the remnants of old enemy clans who see Naruto as the last obstacle to their full control. For example, some rogue members of the Orochimaru - inspired groups might want to capture Naruto to extract his unique abilities since he's the last of his kind.
Well, R2 - D2 and C - 3PO could be characters that Naruto interacts with. R2 - D2 could help Naruto understand some of the technology in the Star Wars universe with his beeps and whistles. C - 3PO could translate things for Naruto if he needs to communicate with different species. And then there's Finn. Finn could team up with Naruto, both being characters who are sort of outsiders in their respective worlds. They could fight side by side against the common enemies of the dark side forces in the Star Wars universe.
Another possibility is that Naruto himself could be his own antagonist in a sense. His loneliness and the weight of being the last man on earth could drive him to the brink of madness. He might have to fight his own inner demons, which could be manifested as a dark version of himself that wants to give up on the world and just let it decay.
It could be a regular person, like a construction worker. They are used to dealing with tough situations and using their hands to fix things. In the 'last human on earth' scenario, they could use their skills to build a safe haven for themselves, fortify buildings against threats, and find creative ways to get resources. For example, they might know how to set up a water - collecting system from the rain.
I'm not entirely sure as 'lemon' in fanfiction often implies inappropriate content. But if we assume it's a normal fic, perhaps it could be about Naruto being the last male in a world facing some kind of gender - imbalance catastrophe. He might have to take on the responsibility of repopulating the world in a non - vulgar way, like through some kind of genetic research or finding a way to reverse the situation that led to this imbalance.
Sakura could be one of the main characters. Since Naruto is the last male, Sakura, being his friend, might play an important role in helping him deal with the situation. She could use her medical skills to support Naruto in his journey.
One idea could be that the last male character discovers a hidden jutsu that was lost for centuries. He then has to master it in secret while also dealing with the suspicion of the other villages who think he's up to no good. This would create a lot of tension and action in the story.
Maybe the main antagonist is a corrupted military leader in the Metro Last Light world. He sees Naruto as a threat to his power and control. He starts a war against Naruto and the people who side with him. He uses his military resources and knowledge of the metro layout to try and defeat Naruto, much like how Orochimaru was a constant threat to Naruto in his own world.