Kotosaka is a parrot who serves as a clansman of the Green Clan, known as Jungle, and functions as the primary communication medium for the Green King Nagare Hisui. While outwardly resembling a typical green-feathered bird, Kotosaka possesses human-level intellect and a small measure of supernatural power granted by his connection to the clan. His most distinctive trait is the ability to channel his king's voice: whenever Nagare wishes to speak remotely, Kotosaka's own voice is replaced by Nagare's, turning him into a living surrogate. This allows the physically confined Green King to address allies, issue orders, and appear before other kings or clans without leaving his refuge.

Within the narrative of K: Return of Kings, Kotosaka appears as a supporting figure who facilitates Nagare's interactions with the wider world. He is often present during strategic discussions or encounters between the Green Clan and its rivals, serving as both mouthpiece and observer. His personality, glimpsed in moments when he is not directly possessed, suggests a playful and cheeky disposition. For instance, he is known to tease the young Green Clansman Sukuna Gojou, engaging in lighthearted verbal spats. Despite this mischievous side, Kotosaka remains entirely loyal to Nagare and the ambitions of Jungle.

Kotosaka's background is not elaborated upon in detail. There is no account of whether he was an ordinary bird changed by the Green King's influence or a strain with innate abilities from the start. What is clear is that he functions as more than a mere pet: he is a recognized member of the clan with a specific role. His ability to speak, reason, and house his king's consciousness sets him apart from mundane animals and makes him a valuable asset in Nagare's plans to seize control of the Dresden Slate. Throughout the series he develops no major arc of his own; instead, his significance lies in how he enables Nagare to maneuver despite his physical limitations.