Taesook is mildly interesting

He's simple-minded. Taesook mainly cares about doing the things he wants and lets his impulses guide him. Unfortunately, he considers girls to be entertainment/objects of lust to satisfy some of those impulses. He tends to pursue fun and interesting things, and Kang Hyomin is fun to hang around. Part of why he seems to treat Gaeul (no makeup) as more of a person is because she's proven herself to have depth by falling into that category. Not only that, but since she's just a "four-eyes" she can't be considered an object of lust. This is the only reason a friendship between the two is possible.