Natsume is regarded throughout Yokohama as one of the strongest ability users. Though the extent of the ability is unknown, I am a Cat allows Natsume to transform into a calico cat, constantly in the form of Kirako Haruno's pet cat named Mii-chan. Moreover, it has been shown that the ability is capable of destroying buildings with relative ease, including an entire Western-style building.
Natsume is rumored to be able to see through all kinds of things, which may be related to Mii-chan's characteristic habit, as noted by Haruno and Naomi Tanizaki, of always wandering off during several catastrophes that were prevented from happening. For instance, they noted that Mii-chan disappeared two days before Yukichi Fukuzawa has been afflicted with Cannibalism. In this event, Mii-chan – who eventually revealed itself to be Natsume – prevented Ōgai Mori from killing Fukuzawa, hence averting a possible crisis between the Armed Detective Agency and the Port Mafia.
The ability is based on the real-life Sōseki Natsume's satirical novel, I Am a Cat (吾輩は貓である, Wagahai wa Neko de Aru?). Set in the Meiji period, the story is told from the perspective of an anthropomorphized cat, a pompous pet cat of a middle-class man and his family. The cat also narrates the interactions of its owner and other characters, including the man's irritating friend and a young scholar who courts the spoiled daughter of a businessman.
- In the anime, a calico cat resembling Mii-chan appeared twice in Bar Lupin during the nightly encounter of Osamu Dazai, Sakunosuke Oda, and Ango Sakaguchi: the night when Ango disappeared and the night when Dazai and Oda confronted Ango about his being a triple spy for the government. Whether this is Natsume transformed or purely another cat merely resembling his feline form is unspecified.
- Interestingly, Dazai referred to the cat as "Sensei", a title often attributed to Natsume by others throughout the series, such as Fukuzawa and Mori.