Suho is described as "tall and foreign-looking," with dark, ash brown hair and light hazel eyes. Though considered very handsome by many, Suho often carries a stoic or cold expression on his face. He has a light, warm-toned complexion. Unlike Seojun, he does not bother styling his hair or dressing up other than on occasions. His clothes are all from high-end brands, as his father is a wealthy, famous actor.
In middle school, Suho was supposedly friendly despite being bullied by his classmates for being a foreigner. After Seyeon's suicide, Suho became cold and expressionless, isolating himself from others and refusing to form new friendships. When approached by others, especially women, Suho gives blunt, curt responses. He only speaks when necessary, and does not consider other people's feelings when interacting with them. After meeting Jugyeong and developing a crush on her, Suho shows a softer, more endearing side. He became generous and giving, initiating conversation with her and smiling more often. Perhaps due to years of isolating himself from others, Suho's social skills are mediocre at best. He envies Seojun's ability to comfortably socialize with people, especially Jugyeong.
Suho is very studious in academics, unlike Jugyeong and Seojun, and spends most of his free time studying. He is fluent in both Korean and Japanese, having lived in both Korea and Japan. He always achieved top marks at school and often tutored Jugyeong. Suho is also an excellent cook, specializing in Japanese cuisine, and occasionally makes food for his friends and sister.
After his estrangement from his father and Seyeon Yoon's suicide, Suho appears to suffer from depression and regularly visits a psychologist for treatment.
Suho is the son of a famous Korean actor, Jooheon Lee, and a Japanese woman, Yuko. He has one unnamed sister whom he occasionally cooks for. After his family moved to Korea from Japan, Suho transferred into a local middle school and was constantly bullied by his classmates for being half-Japanese. Seyeon Yoon, a fellow classmate, became his first friend. Seojun Han saved both of them from bullies and the three became best friends. Suho supported Seyeon and Seojun through their idol trainee days at ST Entertainment. Seyeon eventually debuted through an idol-selection television program, where he faced criticism from netizens.
During this time, Suho's mother passed away from cancer, leaving Suho's family grief-stricken. Jooheon quit acting and succumbed to alcoholism in despair. Two years later, Jooheon returned to the entertainment industry to provide for his family again, promising his children that he would always take care of them. The tabloids started accusing Jooheon of allegedly dating his co-star, which he denied. Suho later caught the two actors together, and in his anger, dropped his phone and stormed out of his home. He sought solace in a nearby comic book shop. That night, Seyeon phoned Suho, who did not answer and committed suicide. Traumatized, Suho lost his friendship with Seojun and moved back to Japan to avoid his father.