Natsume Sōseki (夏目 漱石, Sōseki Natsume?) is a writer and former mentor of Yukichi Fukuzawa and Ōgai Mori. He is at first briefly featured in the light novel, Dazai Osamu and the Dark Era, and makes his full debut in the third season of the Bungō Stray Dogs series.
Natsume is a tall man seemingly in his fifties who always wears a bowler hat, a British-looking, long, brown suit with grey turtleneck shirt underneath, and brown gloves. He is always seen holding a cane. He also has a thin mustache and his hair color matches his fur when in cat form: a light chestnut strand of hair hides the right side of his face, the hair above his neck is white and the rest is dark brown. His eyes are of a brownish-golden color.
When in his cat form, Natsume looks like an average-sized calico cat, with patches of orange, dark brown and white fur and brown eyes.
Natsume is a highly charismatic man; it is assumed that he influenced Ougai Mori and Yukichi Fukuzawa and is even respected greatly by the two. By merely talking to him about a book, he was even able to convince Sakunosuke Oda, a Port Mafia member, to stop killing.
Natsume's ability, I am a Cat (吾輩は猫である (Wagahai wa Neko de Aru?)) allows him to turn into a calico cat. It is commonly said that Natsume is the strongest ability user, able to see through all kinds of things. Until the end of the Cannibalism events, his ability remains unknown to anyone; only then he reveals the nature of his power to Fukuzawa, Mori, and Katai by turning back into his human form.
Not much is known about Natsume's past or whereabouts. He made the creation of the Armed Detective Agency possible by helping Fukuzawa obtain the official permit to run an ability-based business. He is the mastermind behind the Tripartite Tactic, a framework aiming at preserving the balance and order of Yokohama. He acted as a mentor for both Fukuzawa and Mori about twelve years ago, and the two men met thanks to Natsume's input. Sometimes later, he retired.
Natsume repeatedly appears in his cat form as Kirako Haruno's pet Mii-chan.
When Fukuzawa falls gravely ill because of the Cannibalism virus, Natsume, as Mii-chan, vanishes from Haruno's home. He is later seen on several occasions, always in his feline form: once by Tanizaki while he was roaming on the streets; then near Dazai's hospital window, a dried sardine between his teeth; and finally, during the battle between Mori and Fukuzawa. 
While the two leaders are fighting each other, Haruno and Naomi Tanizaki wonder about the disappearance of Mii-chan and how the cat always vanishes before disasters are on the brink of happening. It is later revealed that Katai deceived Fyodor and faked his own death thanks to Natsume's help. Natsume in cat form then passes the information about the Rats in the House of the Dead's hideout's location to Dazai through a dried sardine.
Natsume silently watches as Fukuzawa is defeated by Mori and is bleeding out on the floor. The Port Mafia leader is prevented from killing the President by the sudden explosion of the mansion and the transformation of Natsume who reveals his true appearance. Natsume scolds his two former disciples for being so troublesome, tells them about the discovery of the enemy hideout's location, and urges them to save their lives and prove themselves worthy of the trust he put in them and their respective organizations.