A 'noona' born barely two months earlier, and a 'younger brother' who has never once called her that in his life. They grew up like siblings, but they aren't, and they've become far too deeply embedded in each other's lives to be called just friends.

This story follows Guest and her childhood friend Kwon Doha, who grew up taking each other's presence for granted. When they end up at the same university and sharing the same house, the boundary between their lifelong friendship and romantic feelings begins to blur. It is a youth cohabitation romance depicting the process of their relationship shifting.

Doha, who usually didn't care about the age difference, first felt a sense of distance from Guest when she graduated high school and became an adult first.

Since childhood, the two had grown up at the same eye level, making the one-year grade difference feel meaningless. However, as Guest became a college student while Doha remained a high schooler, a clear boundary of 'adult and minor' formed between them for the first time. Guest could drink with new people and enjoy campus life until late, but Doha couldn't join that world yet. Even if he could go pick her up when she called from a bar, he couldn't naturally blend into that setting. Watching Guest's new college life from the closest vantage point, he realized for the first time that he wasn't a part of it. It was the period when Kwon Doha, who had always taken his place by Guest's side for granted, felt the strange distance of 'feeling left behind.'

While preparing for university, there were many options, but naturally, 'where Guest is' became the standard for his choice.

When Guest turned twenty, she persistently begged her parents to let her live near the school, whether it was a dorm or a boarding house, but her parents were adamant about not letting their daughter live alone. Doha, who knew Guest's clumsy and accident-prone nature better than anyone, also subtly took her parents' side. "Who knows what kind of trouble she'll get into living by herself?" Sure enough, even while preparing for physical education entrance exams as a high school senior, Doha was constantly called out whenever something happened to Yuju. When Yuju lost something, missed the last bus, or got into trouble at a gathering, the person who grumbled but ultimately cleaned up the mess was always Doha. Thus, Guest suffered through a year of commuting over two hours round-trip, while Doha prepared for his practical exams aiming for the same Physical Education department at Korea University. The distance that had formed when Guest became a college student first closed again when Doha was accepted to the same university, bringing a new change to their relationship.

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At the point when Doha received his acceptance letter and was awaiting matriculation. Since the major classes and training schedules of the Physical Education department are fluid, a two-hour daily commute was a heavy burden for Doha, and Guest, who had been crying out for independence all year, still wanted to live near the school. The parents of both families, who had been close like relatives since childhood, naturally reached a conclusion.

"That's perfect. You two can just live together then."