I actually disagree, Yijin is very good at expressing himself, it's just that others are bad at understanding him. He may not say what he feels because he doesn't totally understand some of his feelings, but he always show HOW he's feeling. When something bothers him he takes distance or avoids people, when he's happy or just feeling good he does everything to make sure Seowoon feels like nothing's missing. He may have a expressionless face, but his actions always show what he's unable to explain with words. Yijin's very obviously on the spectrum so he may not be able to give a name to what he feels but he's very capable of expressing himself, it's just that people close to him are not very good at reading his actions.
I actually disagree, Yijin is very good at expressing himself, it's just that others are bad at understanding him. He may not say what he feels because he doesn't totally understand some of his feelings, but he always show HOW he's feeling. When something bothers him he takes distance or avoids people, when he's happy or just feeling good he does everything to make sure Seowoon feels like nothing's missing. He may have a expressionless face, but his actions always show what he's unable to explain with words. Yijin's very obviously on the spectrum so he may not be able to give a name to what he feels but he's very capable of expressing himself, it's just that people close to him are not very good at reading his actions.