Isshuu Tsukimi, representing Monday as the eldest daughter in a family embodying the seven days of the week, is eleven years old and in sixth grade. Following the death of her parents during a trip gifted for their anniversary, Tsukimi shoulders significant caregiving duties for her six younger siblings: Kaji (ten, Tuesday), Mizuo (nine, Wednesday), Mokusuke (eight, Thursday), Kintaro (seven, Friday), Tsuchimaru (six, Saturday), and Hiyoko (five, Sunday). She manages household tasks and provides emotional support, facing ongoing challenges despite her competence in domestic chores and academics. As the oldest, she coordinates the siblings' collective efforts to maintain their household and survive without adult supervision. Visually, she has brown hair and sometimes wears a headband. The parents' death stems from a plane crash in the manga and overwork in the anime; regardless, Tsukimi's story centers on navigating grief and responsibility alongside her siblings, emphasizing familial solidarity and resilience through adversity.