Lemon could be a healer in the village. When Naruto returns, she (assuming Lemon is female) is smitten by his handsomeness. But she also realizes that Naruto has some hidden injuries from his travels. So, she uses her healing skills on Naruto while also developing a relationship with him, and together they face the challenges in the village.
Well, it could be about Naruto coming back to the village after a long journey and he meets a mysterious character named Lemon who is charmed by his newfound handsomeness. Maybe Lemon has some special powers that Naruto needs to help him with a new threat in the village.
Sasuke could be involved. He might be initially against Naruto's plan to capture Lemon, thinking it's a waste of time. But later, he realizes the importance and joins Naruto. Sakura could also be there. She might use her medical ninjutsu to help Naruto if he gets injured during the capture process. Kakashi could be a guiding figure, providing Naruto with strategic advice on how to approach Lemon.
I'm not sure specifically who Lemon is without reading the fanfiction. It could be an original character created by the author for this story, perhaps someone with a special connection to the seal master or with unique abilities related to the Naruto world.
Perhaps a powerful ninja from another village who has a vendetta against Konoha and the ANBU. They see the ANBU's connection with Naruto as a threat and use their knowledge of the lemon fanfiction element in some way. For example, they might create clones that are disguised as lemon - scented ANBU members to infiltrate Konoha and cause chaos.
Maybe a powerful warlord who rules over the region where the winter lemons grow. He sees Naruto as a threat to his control over the valuable lemon resources and decides to eliminate him. The warlord might have an army of minions and some dark magic of his own that he uses against Naruto and his friends.
Obviously Naruto himself would be a main character. He could be joined by Sasuke, who might have his own reasons for being interested in the underworld. For the 'lemon' aspect, Sakura could be involved as a love interest for Naruto. Also, characters from the underworld like a powerful underworld lord or a female warrior who catches Naruto's eye could be main characters.
It could be someone who also wants the power of the mask. Maybe a rogue ninja who has been researching the mask for a long time and sees Naruto as an obstacle to obtaining it.
Maybe the main antagonist is an ancient spirit that has been awakened by Naruto's experiments with transformation. The spirit doesn't like Naruto's interference with nature's balance and decides to punish him by making him turn into a fox with the strange 'lemon' aspect. This spirit then tries to keep Naruto in this form permanently and drive him mad, so that Naruto can no longer be a threat to the spirit's plans of ruling over the ninja world.
Perhaps an ancient evil spirit that was sealed away long ago in Naruto's world but gets awakened by Ruby Lemon's presence. This spirit has the ability to possess people and use their skills for its own ends. It starts to cause chaos in the ninja villages and targets Ruby Lemon and Naruto specifically, seeing them as the key to either its complete destruction or its total domination.
A really interesting antagonist could be a mysterious figure from a hidden organization. This person has been secretly observing Naruto's family for a long time. They have a master plan that involves using Naruto's daughter as a key component. Maybe they want to use her unique chakra or abilities for some dark purpose. They would be constantly manipulating events behind the scenes, setting up traps and challenges for her, all while remaining in the shadows, which would make them a very menacing and unpredictable foe.