Yoon Ji-seok

Yoon Yoo-sun's younger brother and a PE teacher at the high school attended by the Ahn siblings and Kim Ji-won. He is simple-minded, ignorant, and hot-tempered. Unlike most people, his emotional spectrum is very short; he gets angry quickly and cools down just as fast. However, he is warm-hearted, righteous, and athletic. Despite his good physique, he is quite clumsy. Beneath his tough exterior, he is a pure, child-like character. He is dating Park Ha-sun.

Ahn Jong-seok

The son of Ahn Nae-sang and Yoon Yoo-sun, and the brother of Ahn Soo-yeon and Ahn Soo-jung. Once a star ice hockey player, his status plummeted to being the lowest-ranked student in school. He is naturally handsome with superior height and proportions. He planned to go to college as an athletic recruit, but after that fell through, he couldn't keep up with his studies. Although he is a senior, he was held back to a junior class. He is always cynical, stoic, and prickly, but he occasionally makes humorless puns or shows absurd sides. He has a tsundere side and likes Kim Ji-won.

Ahn Soo-jung

The youngest daughter of Ahn Nae-sang and Yoon Yoo-sun, Ahn Jong-seok's younger sister by one year, and Ahn Soo-yeon's twin (born 5 minutes later). She was studying abroad in the US but returned to Korea after the family went bankrupt. She has a boyish and easygoing personality, getting along with her father like a best friend and hanging out well with guys. However, she is stubborn and tough, maintaining a realistic sibling rivalry with Ahn Jong-seok. She frequently uses the word "Stupid" to put others down or insult them.

Park Ha-sun

Ahn Nae-sang's neighbor and Ji-won's cousin who lives in Ji-won's house. She is a Korean language teacher at the same high school as Ji-won and the Ahn siblings. She is kind and soft-hearted, often getting taken advantage of because she puts others first. She dislikes Ji-won riding a scooter. Aside from her kindness, she is clumsy but surprisingly strong.

Kim Ji-won

Lives next door to Ahn Nae-sang and is the owner of the house connected by the tunnel. She is Ha-sun's cousin. Her parents passed away early, so she treasures her phone filled with memories like a prize. She ranks 2nd in the whole school due to her hard work. However, she is more interested in arts and physical education than academics, spending her days off on those hobbies. She is quick-witted, perceptive of others' feelings, and mature. She is very considerate of those around her.

Baek Jin-hee

A college senior struggling with various part-time jobs and student loans. She was fired from an internship and kicked out of her dormitory for certain reasons. While wandering with nowhere to go, Ha-sun offered her a place, and she is now staying at Ji-won's house. She is honest and bright with her emotions but tends to be scatterbrained. Despite her poor and difficult life, she tries hard not to burden others and makes sure to repay any favors. She likes Yoon Kye-sang.

Kang Seung-yoon

Jong-seok's best friend who practically lives at Jong-seok's house. He brings laughter with his unique dialect and quirky, friendly charm. He always carries a guitar and sings self-composed comic songs with explosive vocal talent. He is smitten with Jong-seok's sister, Soo-jung, and praises her smile, though he still treats her mostly as his friend's younger sister.

Song Bo-hyeop

Soo-yeon's childhood friend who has a one-sided crush on her. With his handsome looks, superior height, physique, grades, sports skills, and middle-class background, he is perfect in every way and very popular. However, he has a cold and indifferent personality, ignoring everyone except Soo-yeon. While realistic and prickly to others, he becomes soft and caring only for the easygoing Soo-yeon. He is the only person Soo-yeon feels comfortable opening up to. Whenever he sees Soo-yeon wearing a short skirt, he nags her even before Ji-seok does.