Perhaps she is a civilian girl in Konoha who Naruto has a crush on. She might seem sweet and innocent at first. However, she is actually very self - centered. When a more powerful or wealthy man shows interest in her, she abandons Naruto, leaving him heartbroken.
She could be a kunoichi from a small, unknown village who comes to Konoha on a mission. Naruto, being the friendly and trusting person he is, befriends her. But she has a hidden agenda and uses Naruto's feelings for her own ends, breaking his heart in the process.
It could be that the girl is someone Naruto has known for a long time, like a childhood acquaintance. She might have always been jealous of Naruto's growing popularity and power. So, she decides to play with his feelings. She pretends to love him and then dumps him publicly, making Naruto feel humiliated and heartbroken.
Another possibility is that there is no real 'antagonist' in the traditional sense. Instead, it could be the circumstances that act as the 'antagonist'. For example, the differences in their worlds, their duties, and the fact that they come from such different backgrounds could be what causes Erza to break Naruto's heart.
Hana could be an original character created for this fanfiction. Maybe she is a new ninja in the village who Naruto falls for, but she has a hidden agenda that ends up breaking his heart.
Another possibility could be Sakura. Although she had feelings for Sasuke for a long time, in a fan - created scenario, things could have taken a different turn and she could end up in a relationship with Naruto and get pregnant. However, this would be quite a departure from the original plot dynamics.
In this fanfiction, it could be that Akeno chooses someone else over Naruto. She might publicly reject Naruto in a really humiliating way, like in front of all their friends and allies. This would definitely break Naruto's heart as he is a character who values relationships deeply. He would be heartbroken not just because of the loss of a romantic interest but also because of the feeling of being publicly shamed and rejected.
Well, it could be that Anko does something really cruel like betraying Naruto's trust. Maybe she lies to him about something he deeply cares about, like his dreams of becoming Hokage. And this lie shatters his hopes and thus breaks his heart.
Well, it could be that Hinata starts to ignore Naruto for some reason, like she falls in love with someone else. Maybe she gets influenced by some external force or false information that makes her believe Naruto isn't good for her. This sudden change in her behavior towards Naruto would break his heart as he might have deep feelings for her.
Well, it could be that Hinata might break Naruto's heart by choosing someone else over him. Maybe she realizes she has feelings for another character, like Sasuke perhaps. This would be a huge shock to Naruto as he has always seen Hinata as a special person in his life.
Perhaps Ino lies to Naruto about something really important, like leading him on with false hopes of a relationship. She could make him think she has feelings for him, but then suddenly reveals it was all a joke or part of a mission. This would shatter Naruto's heart because he's so sincere in his emotions. He'd feel used and betrayed.
One consequence could be that Hinata tries to move on but struggles a great deal. She might have a hard time opening up to anyone else in the future. Her self - confidence could also take a big hit. Since she put so much of herself into her feelings for Naruto, having her heart broken could make her question her own worth. Maybe she starts to think that if Naruto didn't love her, then no one will. This could lead to her being more reserved and less likely to take risks in relationships or in other aspects of her life.