Born in the northern mountains, the white ermine Kojopii was eventually sold in an exotic pet shop. Determined to gain freedom, the ermine engineered a successful escape, though the attempt left the animal unconscious inside a garbage can. There, university student Haruka Tsuchiya discovered the ermine and, mistaking it for a ferret, brought it home.

The ermine began a new life at an apartment complex, where a young neighbor, Yuuta Kudou, gave it the name Kojopii. Among other animals, however, the character primarily goes by Okojo-san. Another human resident, Saeki Shun, thought the animal was a mouse and uses the nickname “Chuuchun.”

Kojopii exhibits a confident, aggressive, and proud personality, frequently seeing humans as either servants or rivals. A strong sense of self-importance and a quick temper define the character, often resulting in melodramatic reactions. Despite an outward disdain for humans, Kojopii developed a growing attachment to the extended family of humans and animals in the building.

Much of the character’s experience revolves around struggling to adapt to human society, including encounters with household appliances like vacuum cleaners and interactions with eccentric tenants and their pets. The comedy derives from absurd situations and a tendency toward melodrama.

Notable animal relationships include Chorori, a timid and polite pet mouse who regards Kojopii as a superior. The character also meets a female ermine named Kojoru and a male ferret called Tatchin.

Kojopii’s background traces back to the northern mountains, followed by captivity and a strategic escape from the pet shop. After being discovered by Haruka Tsuchiya, life at the apartment complex began. Over time, the character’s initial proud resistance to human companionship softened into genuine, if sometimes grudging, care for the apartments' extended family.