Lilianne is the twin sister of Amane Todoroki, conceived together but born without a physical body due to complications during gestation. She exists as a disembodied spirit, inhabiting and communicating through a stuffed bear doll. Lilianne possesses an extremely potent form of Incomprehensible Skill of Human beings (ISH), granting her vast supernatural abilities. Lacking a corporeal form prevents her from independently wielding these powers; instead, she channels them through Amane, augmenting her sister's weaker ISH for use in magic performances with their group, Nought.

Her primary motivation involves spreading joy and acceptance of ISH users through these performances, seeking purpose for her existence. Lilianne's relationship with Amane centers on deep interdependence and protection, often reassuring her during crises. This dynamic becomes critical facing threats from the rival group IAM. During confrontations, Lilianne can temporarily merge with Amane, enabling full access to her powers. This fusion carries significant risk, as prolonged integration threatens to erase Lilianne's consciousness permanently.

In the first major conflict, Lilianne initiates fusion to rescue Amane's friend Kozumi from a berserk ISH user, revealing her true spirit form afterward to plead for Amane's acceptance. Later, against the antagonist Aya, Lilianne again advocates for fusion despite knowing the danger, helping Amane break through Aya's illusions by revisiting traumatic memories. Following this event, Lilianne temporarily ceases communication, causing Amane to fear her dissolution. Emotional distress ultimately revives her connection to the bear vessel, allowing her return. After Aya's redemption, Lilianne continues supporting Amane in performances, her existence stabilized but still reliant on the bear.