Let's just talk about the shy boy/confident boy for a minute:
Most people, i would venture to say, have moments when they feel confident, and moments when they don't.
90% of the time, Kattar doesn't (didn't) have any doubts about himself, physical appearance-wise, however, his confidence wasn't unshakeable, as his mother happened to be in the process of turning his crush into a miniature (very miniature) version of herself, which would be terrifying, even if your mother wasn't HIS mother. He was raised to believe that being a man inherently made him a bad person, and he had to do everything in his power to counteract that. So while he can interact with his bff/crush pretty calmly, and even charismatically, most of the time, when the time comes for feelings, it's not a pretty picture, because he's not sure if she's finished the metamorphisis into a woman who believes men are a waste of time and energy.
Now Jinwoo?: not sure he understands the concept of being awkward fully. He is himself, he's been told he's weird (in so many words) or that other people might be afraid of him, but he doesn't know WHY. HE thinks that he's normal, and that other people are kind of weird, but the fact that he's seen first hand how uncomfortable he makes people still leads him to try to be quieter and supress his thoughts and behaviors so he doesn't scare them away.