Himari Takakura is a central figure known for being the terminally ill younger sister of Kanba and Shoma Takakura, with whom she lives. Despite having no blood relation to her brothers, she is a cherished member of the family, having been taken in after being abandoned by her own mother. From a young age, Himari has suffered from a grave, unspecified illness that leaves her physically weak and prone to fevers and fainting spells. She lives under a constant shadow of death, often confined to hospitals and her home.

To shield her beloved brothers from sorrow, Himari maintains a perpetually cheerful, optimistic, and sweet demeanor. She is an enthusiastic and sometimes mischievous girl who dreams openly and refuses to let her condition rob her of joy. Her kindness is deeply empathetic, making her highly perceptive to the feelings of others, and she possesses a nurturing, almost maternal side, becoming highly skilled at cooking, cleaning, and knitting from her long periods at home.

The story takes a dramatic turn when, after being discharged, she dies during a trip to the aquarium. Her life is miraculously extended by a mysterious spirit, the Princess of the Crystal, who resides in a penguin-shaped hat purchased at the souvenir shop. When the hat is placed on her head, Himari transforms completely. Her appearance shifts to a skintight, dominatrix-style outfit, and her personality is overridden by that of the Princess—an arrogant, vulgar, and commanding individual who calls for Survival Strategy and forcefully orders the brothers to find the elusive Penguindrum to keep Himari alive. This hat can transport others to an alternate dimension for these encounters, though the victims later perceive it as a dream. Her motivations are rooted in a desperate wish not to burden her brothers, whom she loves deeply, while her alternate persona is aggressively focused on her own survival. Before her illness isolated her, Himari was a member of the aspiring idol group Triple H with her friends Hibari and Hikari, and she later expresses genuine happiness for their success.

Within the Takakura family, Himari is the emotional core, the person both Kanba and Shoma sacrifice everything to protect. Her relationship with them transcends biology, anchored by the concept of the fruit of fate; as a child destined for the Child Broiler, a place for those who will never amount to anything, Himari was saved when Shoma shared his portion of the fate fruit with her, intertwining their life forces. This shared fate creates a cycle of sacrifice where the brothers pay the cost of her borrowed time. Himari is not merely a passive victim; she is aware of being a burden, and her development involves a quiet rebellion against this role. In a critical moment, she willingly gives back her share of the fruit to break the cycle of her brothers sacrificing themselves.

Ultimately, the shared life force, the love among the siblings, is the Penguindrum itself. Himari survives the final ordeal, but at the cost of her brothers being erased from existence to change fate. In a new worldline, she lives without memories of them, yet an inexplicable sadness leads her to tears when she sees traces of her lost family. Himari herself has no combat abilities or supernatural powers. Her only magic comes externally from the penguin hat, which contains the Princess of the Crystal. Her notable talents are entirely domestic: she is an excellent cook, capable of all household chores, and has a remarkable ability to knit scarves and vests.