Lumi is the younger sister of Laila, daughter of the logistics magnate Valentin. She is physically frail, having an autoimmune disorder that prevents her body from regulating its temperature properly, requiring her to wear winter clothing year-round. Despite this, she hates being treated like a piece of glass and insists that everyone treat her as a normal girl, even though she doesn't really know how to act like one.

Although she was named for her pure white hair, her name turned out to be a terrible irony due to her condition and resulting hatred for all things cold.

If the player completes Laila's questline and procures a cure for Lumi's condition, she will become much more upbeat, and her outfit will change with the seasons.