Nyanko-sensei, originally named Madara, is a powerful flying white wolf daiyokai. He typically manifests as a plump maneki-neko with white fur, orange and grey markings, black eyes, and a red collar bearing a bell. This form renders him visible to humans lacking spiritual perception, though he vehemently denies being an actual cat despite frequently exhibiting feline behaviors such as chasing small animals and playing with cat toys. His true form is a colossal wolf-like spirit with white fur, dramatic red facial markings, and piercing gold eyes, radiating immense spiritual power that instills fear and respect in lesser yokai. His abilities include flight, shapeshifting between his cat and wolf forms, projecting convincing human disguises (notably resembling Takashi Natsume or Reiko Natsume), and firing destructive spiritual light rays from the marking on his forehead.

Long ago, an unidentified powerful exorcist sealed Madara inside a maneki-neko statue, imprisoning him for decades within a forest shrine. Takashi Natsume accidentally freed him by tripping over the sealing rope while fleeing a yokai. Madara initially attacked Takashi to seize the Book of Friends—a ledger containing the names of spirits bound by Takashi's grandmother, Reiko. However, he relented upon witnessing Takashi's fierce determination to cherish the book as his sole link to Reiko. Madara negotiated a pact to serve as Takashi's bodyguard in exchange for inheriting the book upon Takashi's death. He subsequently moved into the Fujiwara household as Takashi's "pet," adopting the nickname Nyanko-sensei while insisting he is the true master.

Madara exhibits a gruff, condescending, and prideful personality, frequently complaining about Takashi's kindness towards spirits and humans. He is selfish and comedic, prone to immature behaviors like overindulging in sake, consuming sweets (especially dangos) gluttonously, and stealing food. Despite his protests, he displays distinct cat-like mannerisms, bullies dogs, and throws tantrums. Secretly, he develops a deep fondness for Takashi, becoming fiercely protective during life-threatening situations, transforming to defend him from exorcists or dangerous yokai. He similarly protects the Fujiwara home when threatened, motivated partly by appreciation for Touko Fujiwara's cooking. This attachment extends beyond their contract, evidenced by his regret when Takashi rejects an immortality that would delay his inheritance of the Book of Friends.

In "Itsuka Yuki no Hi ni," Madara accompanies Takashi during a snowy encounter with a distressed snowman yokai searching for a lost item. He grumbles about Takashi's involvement but remains by his side, using his powers to intimidate the spirit when it invades their room. He dismissively swats away a frozen artifact the snowman carries, inadvertently revealing its contents—a box of Shigeru Fujiwara's childhood toys. This incident highlights Madara's role as a reluctant yet steadfast guardian, observing parallels between the snowman's quest and human tendencies to seek intangible comforts.

His relationships evolve: with Takashi, their bond shifts from mutual distrust to familial loyalty, including moments where Takashi protects an injured Madara. He distrusts exorcists like Shuuichi Natori and fears Seiji Matoba's sinister aura. Among spirits, he maintains alliances with figures like Hinoe and Benio while intimidating lesser yokai. His history with Reiko remains enigmatic but intimate, though he denies ever losing to her. Madara's development culminates in prioritizing Takashi's safety over the power offered by the Book of Friends, transcending his initial selfish motives.