Kate Takayama is a supporting character in the Haganai NEXT storyline, known primarily as the older sister of Maria Takayama, the young nun who serves as the faculty advisor for the Neighbors Club. Despite her age, Kate holds a position as a nun at St. Chronica's Academy, a role she shares with her younger sibling. She is a 16-year-old girl with a striking appearance that has been described as a matured version of Maria, sharing her sister's lavender-silver hair, blue eyes, and pale skin. Unlike Maria, Kate possesses a well-endowed, curvy figure that makes her appear older than she actually is, along with a distinctive beauty mark located under her left eye.

Her personality presents a sharp contrast between her professional religious role and her personal behavior. While she can be composed and serious on the surface, giving off an impression of maturity that can confuse people about her true age, she also has a notoriously vulgar side. Kate is not embarrassed about openly farting, scratching her butt, burping, or telling dirty jokes in public, which stands in stark opposition to her image as a nun. She holds an eccentric viewpoint on life and religion, even describing Christianity as a rebellious faith. However, when presented with food, her demeanor completely transforms, and she adopts a simplistic, excited manner of speaking that is very similar to her younger sister's personality, coupled with what seems to be an even more gluttonous appetite.

The core of Kate's motivation and character revolves around her relationship with her sister Maria. Kate has taken on the role of a parent due to unknown circumstances, acting as the primary caretaker and disciplinarian in Maria's life. She is overprotective to a considerable degree, famously calling Kodaka every hour during a sleepover to check on what Maria ate and whether she had brushed her teeth. While she frequently scolds Maria for her childish antics and forces her to eat healthy food, she genuinely cares for her sister's happiness. She is deeply grateful to Kodaka and Yozora for involving Maria in the Neighbors Club, noting that they have been able to cause a positive change in Maria that she and the other sisters could not accomplish. At the same time, she shows jealousy regarding Maria's affectionate, sibling-like relationship with Kodaka and her friend-like dynamic with Kobato.

In the story of Haganai NEXT, Kate is introduced as a character who interacts primarily with the members of the Neighbors Club. Her relationship with Kodaka Hasegawa is notably complex. She possesses a flirtatious side, often teasing him and calling him onii-chan, and there are hints that she harbors romantic feelings for him. However, due to her knowledge of the arranged engagement between Kodaka and Sena Kashiwazaki, she implies that she would not pursue those feelings further. She also maintains a friendship with Sena's father, Tenma Kashiwazaki, as they are fishing buddies, and it is through her conversations with him that she learns about the engagement. Her relationship with Yozora Mikazuki is one of gratitude, as she appreciates that Yozora was the first person to simply see a child in Maria rather than a genius prodigy and interact with her accordingly.

Kate possesses several notable abilities that support her role within the academy. She has a vast knowledge of Christianity, including its origins and history, which she draws upon in her duties as a nun. Given her young age and the fact that she holds a position of authority within a school environment, she is also implied to be quite intelligent, with some students even addressing her as sensei. Her development throughout the series is subtle but significant, as she transitions from a purely overprotective sister who tries to control Maria's environment to a more supportive figure who recognizes the value of Maria's experiences with her friends, ultimately learning to let her sister grow independently.