The important characters in Black Zetsu's story are numerous. Kaguya Otsutsuki is the most fundamental as Black Zetsu was born from her will. Madara Uchiha is a central figure as he was deceived by Black Zetsu. Madara thought he was working towards his own goals, but in reality, he was just a pawn in Black Zetsu's plan. Naruto Uzumaki and Sasuke Uchiha are vital as they were the ones who had to face Kaguya in the end. Their power and determination were what stopped Kaguya's resurrection from being a complete success. Additionally, characters like Obito Uchiha, who was involved in some of the events related to the Akatsuki, which Black Zetsu also had a hand in, are also related to Black Zetsu's story.
There could be internal antagonists within Naruto's own village. Some of the more conservative ninja in the village might not trust Naruto's new - found skills and powers obtained from Zetsu. They could try to undermine Naruto or even attempt to exile him, fearing that he has been corrupted by Zetsu's teachings.
Well, one way could be in their physical abilities. Maybe in the fanfic, Naruto is shown to have some of Zetsu's stealthy movement skills, like being able to blend into the environment in a similar way. Another similarity could be in their connection to nature. Zetsu has a strong link to the plant - like aspects of nature, and Naruto's chakra might be shown to have a similar kind of connection in the fanfic.
Obviously, Naruto and fem Zetsu are the main characters. Naruto, with his determination and dream of becoming Hokage, is the protagonist we all know. Fem Zetsu, on the other hand, brings her own unique set of abilities and a different perspective, being a part of the Zetsu race which has its own mysteries in the Naruto universe.
Naruto himself is a great choice. Their personalities are so different that it can create a lot of interesting interactions. Naruto's positive and determined attitude can contrast with fem Zetsu's more mysterious nature.
Perhaps a new, powerful and mysterious ninja who has a vendetta against the Hidden Leaf Village where Sakura is from. He sees Zetsu as an accomplice of the village and targets both of them, causing chaos and danger in their lives. This new character could bring in a fresh and unexpected element of danger to the fanfic.
Sasuke would be a likely character to interact. Given their complex relationship throughout the Naruto series, Sasuke might be suspicious or intrigued by Naruto's control over Zetsu. Their interactions could range from confrontations to reluctant teamwork.
Naruto could be the one to save her. He has a strong bond with Sakura and would never let her be in danger for long. His powerful ninja skills and his unwavering determination would make him a likely candidate to rescue her from Black Zetsu's grip.
I'm not sure specifically as there could be many different interpretations in fanfiction. But it might involve Zetsu using his unique abilities to rescue Naruto from a dangerous situation, like a powerful enemy attack or a life - threatening trap.