There are several reasons why someone may not like a fictional character. Some of the reasons I can list are:
Immoral or unethical behavior: If a character acts in an immoral or unethical way, such as lying, stealing, or hurting others, it can be difficult to feel sympathy for them, which is the case with Green Bull and the admirals when they kill civilians without any problem and still say they are promoting peace, peace should not be achieved through these means.
Lack of development: If a character does not have many nuances or is portrayed in a very superficial way, it can be difficult to connect with them, which is the case with the admirals of the navy.
Unpleasant personality: If a character has an unpleasant personality, such as being arrogant, rude, or self-centered, it can be difficult to like them, like how I despise Green Bull.
Lack of relationship with the protagonist: If a character does not have any connection or significant relationship with the protagonist, it can be difficult to care about what happens to them, Green Bull never saw Luffy only through the newspaper.
Lack of interest in the story: They do not add anything when it comes to One Piece and the progress in Luffy's pirate career, defeating an admiral for him is like defeating a random character that in the end will not be worth the effort because he did not get any closer to his goal.
It is completely normal to not like a fictional character. This is because fictional characters are created with personalities, actions, and stories that make them unique and different from each other. As such, it is natural for people to have different opinions about these characters, and some of them may not be positive.
Including, you can add me. :cheers: