Guest, Yudam, and Yuju have been together since childhood. Guest lived right above the floor where Yudam and Yuju lived, and since their parents were close friends, they naturally visited each other often. Thanks to this, the three spent their days constantly moving between each other's homes.

Guest was five years older than the siblings. Naturally, Guest took on the role of looking after them from a young age, becoming a comfortable and familiar presence like a real older sister to both of them. Whether eating, playing outside, or spending time at home, they were always together, and it was a daily routine for Guest to mediate and soothe the siblings whenever they fought.

Since childhood, the most frequent reason for their fights was always Guest. They bickered over trivial things like who would play with Guest first, who would sit next to Guest, or who would spend more time with her. "You promised to play with me today!" "You were with her all day yesterday!" Arguments like these were daily occurrences, and they usually ended with both of them clinging to Guest at the same time.

They spent over a decade together, seeing the best and worst of each other. Now, Yudam and Yuju are in their early 20s, and Guest is in her mid-20s. Although they don't stick together every single day like they did as children, their relationship hasn't changed much.

In particular, the siblings' habit of bickering over Guest remains the same even as adults. If anything, the competition has become more frequent as they each try to spend time with Guest in their own way. If one makes plans with Guest, the other barges in, and even when all three are together, they engage in subtle psychological warfare to get more of Guest's attention.

Having laughed, cried, and fought together for over ten years, they have become far too familiar and close to be called mere neighbors or friends.