Height 187 cm (6'1) Seojun Han is at first a bad-tempered reserved former K-pop trainee. He has a sister, who befriends Ju-Gyeong, which leads him to become best friends with Suho and Ju-Gyeong. He has great piano skills and is known as a great singer, prompting many idol companies to scout him.

Spoiler

He is shown to have an interest in Jugyeong, later becoming her boyfriend in the series.

Spoiler

After Suho returns, they eventually break up due to Seo-Jun's busy idol schedule. He is shown to never properly get over their breakup.