Another possibility is that society or the shinobi world's expectations act as the antagonist. If Ino breaks Naruto's heart because she feels pressured by the norms of their world, like not being allowed to be with someone of a different clan or status, then those unwritten rules could be considered the antagonist.
It could also be some outside force that's influencing Ino to break Naruto's heart. Maybe an enemy has some sort of control over Ino's mind or has threatened her into doing it. In this case, that outside enemy would be the antagonist, while Ino is just a tool being used.
Ino could be seen as the antagonist in this fanfiction. Since she's the one breaking Naruto's heart, she's causing the main conflict. Her actions are directly opposed to Naruto's happiness, which is a typical trait of an antagonist.
Maybe it's a jealous rival of Naruto who wants Hinata for himself. He manipulates situations in such a way that Hinata ends up hurting Naruto. For example, he could stage events to make Hinata think Naruto has betrayed her trust.
It could be Sasuke. He has had his own dark journey and might turn against Naruto and Ino in this fanfiction. His newfound power and different ideals could make him see Naruto and Ino as threats to his goals.
Perhaps there is no traditional antagonist. Instead, Naruto's own insecurities could be the 'antagonist'. He's so used to wearing the mask of the happy - go - lucky guy that his inner self is resistant to being exposed. So, in a way, his own fear of change and vulnerability is what Sakura has to fight against when she tries to break his mask.
There could be many unknown authors out there writing fanfictions. It's hard to say specifically who the author of this one would be without more research. It might be some Naruto - loving fan who wanted to explore a more tragic and angsty storyline with these characters.
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.
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.
Another possible antagonist could be the Ministry of Magic. If they suspect Harry of something and force Tonks to spy on him or cut ties with him, it would break Harry's heart. The Ministry's strict rules and paranoia could be the driving force behind Tonks' actions that lead to Harry's heartbreak.
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.
Perhaps they are characters in a modern - day wizarding world where they are both part of a secret magic society. Edward could be a powerful wizard from an old family who feels threatened by Harry's new - found powers. So, he decides to break Harry's heart to assert his dominance.
Sasuke could be one of the characters to help Naruto. Their combined powers have been shown to be very effective in the original series.