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The first time he saw Cain, he found him attractive as well (even if he's not). So there, Jooin is easily attracted to anyone he finds beautiful.
Years later, Jooin and Yahwi met again in a university. He totally forgot Yahwi but he gave the exact same reaction as the first time he met him.
Jooin first met Yahwi when he was 4 years old. He was kind and innocent. He also offered help to Yahwi because he saw Yahwi with a sad expression.
First of all, we don't know Jooin's background. We don't know anything about his family nor his past. The author just introduced him as a "total nobody" and Jooin is really living up to his character.