@Mai_loves_icecream But it wasn't a harmless secret. She literally was forced to spend time with him. Even if she eventually grew to like him, she felt the anxiety of what he was holding over her neck. It was a secret so big, it hurt her best friend, the whole class turned on her, it brought up old wounds from middle school and she was so distressed that she decided to transfer schools. And change her name.
He knew it was a big secret because even he said "you wrote this even though you're friends with her" before he understood the context of the letter. Even he initially questioned what kind of person she was.
It's damaging blackmail. He knew. She knew. And it made her miserable knowing he had that on her. Look through those panels when he finally says he'll give the letter back to her. She's stressed but she decides to trust him anyway. He broke that trust by being very stupidly careless with the letter with a house full of people who know her. He can get angry with glasses as much as he wants but he's ultimately to blame.
Eun Jae cares enough about him to be relieved he didn't personally expose her but I don't really see how it makes a difference.