At Eighteen

Students

• Lee Seung-min as Lee Ki-tae

Class 2-3 student; So-ye's boyfriend and a part of Hwi-young's clique. He keeps himself close to Hwi-young to gain his friendship and be included in private study groups, to the extent that most would see him as Hwi-young's servant.[12]

• Moon Bin as Jung Oh-je

Class 2-3 student; Joon-woo's seatmate and best friend. He is handsome, athletic, and likes to play basketball; he is Da-heen's crush.[12] In his family, he would help his father out by either managing their tteokbokki shop or taking care of his baby brother. It is revealed in later episodes that he is gay; he harbored a secret crush on Hwi-young.

• Kim Do-wan as Cho Sang-hoon

Class 2-1 student. A natural genius, particularly in math, he is sees Hwi-young as his rival in academics.[12]

• Yoo In-soo as Yoo Pil-sang

Class 2-3 student. Rough on the outside, he has his own charms. He has a crush on Ro-mi.[12]

• Baek Jae-woo as Go Dong

Class 2-3 student. He frequently accompanies Pil-sang.[12]

• Woo Joon-seo as Park Kyu-nam

Class 2-3 student. He likes to play mobile games.[12]

• Shin Ki-joon as Ha Shim-bok

Class 2-3 student. He is also a part of Hwi-young's clique.[12]

• Kim Ga-hee as Moon Chan-yeol

Class 2-3 student; one of Soo-bin's friends. Though boyish, she has a crush on their homeroom teacher Mr. Oh.[13]

• Moon Joo-yeon as Yoon So-ye

Class 2-3 student; Ki-tae's girlfriend and one of Soo-bin's friends. She is a ballet dancer.[13]

• Kim Bo-yun as Kwon Da-heen

Class 2-3 student; one of Soo-bin's friends. She has a strong crush on Oh-je.[13]

• Han Sung-min as Hwang Ro-mi

Class 2-3 student; one of Soo-bin's friends. She is jealous of Soo-bin being in the top tier. She has a crush on Joon-woo and would ignore Pil-sang's advances.[13]

Parents

• Shim Yi-young as Lee Yeon-woo

Joon-woo's mother. A single young mother, she runs a restaurant but is in debt. She loves Joon-woo very much.[14]

• Choi Jae-woong as Choi Myeong-joon

Joon-woo's father. He broke up with Yeon-woo when he found out he had accidentally bore her a child (Joon-woo). He has his own family now.[14]

• Kim Sun-young as Yoon Song-hee

Soo-bin's mother. She forces Soo-bin to be top in class and prepare for going to an in-Seoul college. She is in bad terms with her husband Jong-soo, who has now a new woman.[14]

• Lee Hae-young as Yoo Jong-soo

Soo-bin's father. He wants to be divorced with his wife, but he loves Soo-bin as his daughter.[14]

• Jung Young-joo as Park Geum-ja

Hwi-young's mother. Spoiling her son and out of the fear of starting her husband's fury, she would use her wealth to manipulate Hwi-young's teachers.[14]

• Sung Ki-yoon as Ma Yoon-gi

Hwi-young's father. Forcing his son to be top of his class, he puts both his wife and Hwi-young through emotional abuse.[14]

Others

• Park Sung-geun as Lee Kwan-yong

Vice-principal of Cheonbong High School[15]

• Heo Young-ji as Kim Ji-min

An employee at the convenience store where Joon-woo works part-time. She usually calls Joon-woo as "Park Young-bae" (the name written on the second-hand uniform Joon-woo is using at work). She is trying hard to find a better work.[15] She is Mr. Oh's love interest.

• Choi Woo-sung as Im Gun-hyuk

Joon-woo's former schoolmate

• Choi Dae-hoon as Teacher Son

Mathematics lecturer

• Lee Seung-il as Joo Hyun-jang
• Jeon Jin-seo