The unagi's body is long and serpentine, with no scales and minimal aposematic coloration. While uniformly light gray in appearance ventrally, the creature's dorsal surface is primarily dark gray with light gray segments over both cheeks and a light gray stripe extending down the midline of the body. Located posteriorly to the head are four yellow stripes similar in appearance to gill slits.

A large dorsal fin protrudes from the top of the unagi's head, consisting of six spines interconnected by cartilage that allow the creature to expand and contract the structure during movement. Behind the primary dorsal fin are a series of evenly dispersed finlets that gradually increase in size toward the caudal end of the body.

The unagi is a carnivorous predator that will consume anything that it encounters in the water, with a particular preference for the elephant koi found in the waters surrounding Kyoshi Island. It is capable of launching powerful streams of water from its mouth as a means of subduing prey and deterring external threats.

Because of its aggressive nature, the residents of Kyoshi Island typically avoid the bay in which the unagi is found. Despite its tendency to lash out, the unagi has an instinctive fear of anything that exceeds it in size, and it can be controlled, to some extent, by its barbels, as demonstrated by Aang when he forced the unagi to spray water over the burning Kyoshi Village.

The unagi has many attributes found in members of Anguilliformes, particularly to several species of carnivorous freshwater eels also known for their serpentine appearance. Its use of pressurized jets of water to stun prey also resembles the hunting strategy employed by archerfish, which use their specialized jaw musculature to strike terrestrial i...