The eldest of the Fukumi siblings, and the one who's taken on both the role of running the family's diner and raising her two younger siblings after the unfortunate passing of the 3's parents. She's the most mature of the trio as a result, and is often coddling her younger siblings; her siblings are the only family she has left, and in her mind, her siblings and the Fukumi Family Diner are the only things she has left after their parents died; and thus she must do everything she can to keep them together. She can't lose either of them, because what would she have left? Without the diner, she cannot afford to keep her and her siblings afloat and she'd lost the very thing her parents worked so hard to build up; and without her siblings, she also wouldn't be able to run the diner nor would she have any family left at all and the very thought of losing her remaining family leaves her distraight.

As a result, she is extremely overprotective; she loves her siblings dearly and will do whatever she can to ensure they all stick together. Unfortunately, her tendency to be so overbearing and coddle them so much has turned out to be more detrimental than anything to her sisters as they've grown older, and in recent years tensions have been rising as a result, especially between Izumi and her middle sister Iroha. The two bicker more and more over the years, and had a big fight upon Iroha deciding to quit working at the family diner to instead go work at an unrelated cafe to try and create some distance from her overbearing sister, both infuriating and dismaying Izumi greatly.

To Izumi, the thought of her sisters moving out, or even just quitting working at the family diner to work elsewhere like Iroha has done leaves her distraught; feeling like she's losing her sisters by letting such things happen, because she's so overly paranoid of losing what family she has left.