Why does Erica hate Seiya from Sailor Moon?

Answer 1

I'm not sure who Erica is, so it'd be pretty difficult to answer your question. However, several people have various reasons for disliking Seiya, including:

• The fact that her gender was changed to male in the anime; in the manga Seiya Kou was always female and only crossdressed as a male when posing as a member of the popular band Three Lights. Further, this "gender swap," even though it was meant to be part of Sailor Starfighter's "disguise" on Earth while seeking Princess Kakyuu, seemed to alter the personality of Seiya mores than she appeared in the manga
• The fact that, in the anime, "he" is often quite callous and manipulative of Usagi and her affection for Mamoru, who has not been seen since his departure to Harvard University in the United States. In the manga, she didn't let her love for Usagi take precedence over the impending war with Sailor Galaxia, and instead tried to hint to Usagi regarding Mamoru's fate, only being explicit with her when Usagi failed to admit the fact that he had been killed.
• The fact that many fans prefer to pair Seiya with Usagi, when the canonical and widely-accepted pairing for the Sailor Moon series is Usagi and Mamoru.

However, these are just a few possible reasons why someone might dislike Seiya from Sailor Moon; each person is different.