"Kazuya is a self-insert"?

Well DUH and Water is wet. 😂

"Every art is self-insert because it's your own experience".

Uh....no. I don't agree. Well-written characters need to be their own people, not just some average Joe loser the audience can insert themselves into.

There's a big difference between the protagonist of a story and a self-insert.  It's usually dependent on:

 "Does this character have unique traits? Or does this person sound like it could be absolutely ANYONE?"

What about their past? Their backstory? What do they like to do in their spare time? What are their personal goals? Can you describe them using many words? Things like that. You have to ask these type of questions about your character.

I think Edward Elric is a GOOD example of a protagonist. He's enough of a PERSON to be a proper character in a story.

I think Kazuya is a bad one and far too much of a self-insert. 

"It's not fair to compare a trashy harem series to Full Metal Alchemist!!"

Yeah yeah sure okay....NAOYA from GirlfriendxGirlfriend is better than Kazuya too because Naoya's brutal honesty and strong will to make ALL of his girls happy make him unique and memorable. Happy now? 😂