Coji-Coji is an enigmatic cosmic being, the "Child of the Universe," whose origins remain undefined. Possessing god-like abilities such as erasing the universe, firing energy beams, and living millennia, it typically avoids using these powers. Instead, it prefers simple daily activities—eating, sleeping, playing—within the fantastical Meruhen (Fairy Tale Country).

Physically, it appears as a small, light-yellow creature. It lacks conventional hair and is sometimes characterized with a rabbit-like face and a potato-shaped body, not explicitly resembling any known animal. It is identified as a "nature creature" or "space organism."

Its personality blends apparent naivety with profound, often philosophical insights. Its dialogue fluctuates between nonsensical and unexpectedly wise, its carefree demeanor masking deep existential understanding. Coji-Coji frequently makes blunt or socially inappropriate remarks bordering on callousness, causing comedic misunderstandings. Despite this, it remains fundamentally kind-hearted, subtly helping friends without seeking recognition. Companions consistently overlook its divine nature, attributing extraordinary events to coincidence or ignorance.

Relationships center on Meruhen's diverse inhabitants: Jiro (a flightless, aquatic hybrid), Korosuke (a non-melting snowman), Johnny (Meruhen's sole human), and others like the witch Susie and sun-king Geran. Coji-Coji acts as a passive mediator, resolving conflicts through indirect or unintentional interventions.

The character appears across multiple media: the original manga (1994–1997), a 100-episode anime series (1997–1999), a Dreamcast party game (2000), a pachinko arcade game (2010), and a stage play (2019). Its portrayal remains consistent throughout these adaptations, with no significant evolution in background or abilities. A remastered 20th-anniversary anime special was released in 2017.