Noren Mitsuoka is a central character in Kill Blue, serving as the main heroine and deuteragonist. She is a middle school student who comes from immense wealth as the daughter of the founder of Mitsuoka Pharmaceuticals, the powerful corporation responsible for creating the artificial wasps whose venom de-aged the protagonist, Juzo Ogami. In addition to her privileged background, Noren has Canadian heritage from her mother's side. She is an exceptionally beautiful young girl with a fair complexion, long blonde hair that she typically parts on the left, and striking purple or pink eyes. While attending school, she wears the standard uniform, often accessorized with a neckerchief or choker. She later undergoes a notable change in appearance by cutting her hair to neck length.

Noren is a complex character who excels in both academic and physical pursuits, yet her exceptional beauty and poised demeanor create a barrier between her and her peers. She is constantly pursued by male students, all of whom she rejects, which has contributed to her deep aversion toward men. This distrust stems from her family's high-society connections; from as early as elementary school, her parents introduced her to adult men they deemed suitable for marriage. These men treated her as if she were already an adult woman, an experience she found repulsive and unsettling. As she grew older, she came to see boys her own age as exhibiting the same troubling qualities.

Despite her outward coolness and reserved nature, Noren harbors a secret, heartfelt aspiration that runs counter to her family's expectations. She dreams of one day taking over the ramen shop owned by her uncle. This restaurant represents a sanctuary for her, as her uncle is the one person who never gave her strange looks and always provided comfort with a bowl of ramen on difficult days. He shares her disdain for high-society living, and working at his shop allows Noren to envision a different path for herself, one where she can live an ordinary life on her own terms.

In the story, Noren is initially a target for Juzo's investigation, as he believes her connection to Mitsuoka Pharmaceuticals may hold the key to reversing his condition. However, his attempts to befriend her at school are met with immediate rejection, with Noren labeling him as creepy and insisting they interact only when necessary. Their relationship takes a turn when Juzo discovers her working at her uncle's ramen shop. A sincere conversation there reveals her true feelings about her family and her dreams for the future. Juzo, who reminds her of her uncle, encourages her to pursue her own desires rather than conform to her parents' expectations. This encounter marks the beginning of their complex bond.

Over time, Noren and Juzo form a close friendship. After Noren discovers Juzo's true identity as an assassin, she actively chooses to help him. She proposes that he pretend to be her fiancé, a scheme that allows them to infiltrate Mitsuoka Pharmaceuticals in search of a way to reverse his transformation. As they spend more time together in this fabricated relationship, Noren begins to realize that her feelings for Juzo are becoming genuine. Her personal journey is one of opening up to another person for the first time, moving past her deep-seated aversion to men as she learns to trust and rely on Juzo.

In terms of abilities, Noren is depicted as highly capable, possessing impressive natural talents and a good education that allows her to excel in many tasks. However, she tends to struggle with intricate or detailed work. Her true passion and skill are tied to her work at the ramen shop, where she takes pride in helping to prepare the dishes.