There's no way to know without reading it. However, it might be a male ninja who has some kind of special significance. He could be a character who was previously a minor one in the Naruto story but is now thrust into the spotlight as the last male nin. Or it could be a completely new character designed to bring a fresh perspective to the Naruto fanfic world.
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'.
I'm not sure specifically as there could be many different interpretations of such a fanfiction. It might be about the last male ninja in the Naruto universe, perhaps his final adventure, his relationships, or his role in some kind of end - of - the - world scenario in the Naruto - inspired world of the fanfic.
One of the important first male ninjas in Naruto fanfiction is Rock Lee. His determination to become a great ninja despite not having natural ninja talents like chakra control makes him an interesting character. He is often depicted in fanfiction as someone who overcomes great odds. Also, Neji Hyuga, with his complex family history and powerful Byakugan, is another male nin who features prominently in fanfiction.
Maybe a female ninja who has been secretly training to become a Hunter Nin. She could break the gender stereotypes in the Naruto universe and show that female ninjas can be just as effective, if not more, as male Hunter Nins. She could have her own unique set of skills and a strong motivation for becoming a Hunter Nin, like avenging her family who were killed by a rogue ninja.
Danzo could be a great antagonist. He always had his own secretive and often unethical plans. He might see Naruto as a threat even if he's innocent and frame him as a missing - nin to get rid of him.
Obviously Naruto is one of the main characters. His nin animal would also be a main focus. There might also be his friends like Sasuke and Sakura who interact with Naruto and his nin animal in different ways. Maybe Sasuke is initially jealous of Naruto's unique nin animal.
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.
Well, in Naruto fanfiction, Kakashi Hatake can also be considered among the first male nin. He is a highly skilled and experienced ninja. He serves as a mentor figure in the series. His unique abilities like the Sharingan and his cool demeanor make him a popular character in fanfiction as one of the early important male ninjas.
It could be a rogue hunter nin who doesn't like the idea of Naruto, an outsider, joining their ranks. This rogue hunter nin might see Naruto as a threat to their power and influence within the group.
Kimimaro could be an interesting missing nin character to explore in fanfiction. He was a tool for Orochimaru, but had his own internal struggles. You could write about his life before fully becoming a missing nin, his loyalty to Orochimaru despite his own deteriorating health, and what might have happened if he had broken free from Orochimaru's control. Another character could be Nagato. Before he became Pain, he had experiences that could be considered as leading towards a missing - nin - like existence. His views on peace and his use of the Akatsuki could be delved into in the context of a missing nin story.
Sasuke is often a popular one. In fanfiction, his journey as a missing - nin and potential connection to Akatsuki can be explored in many ways. His power and his complex relationship with Naruto make him a fascinating character to write about in this context.