As Team Avatar was resting in the forests outside Makapu Village, a se tu was swimming in a river, diving in and out of the water. His actions were perceived as taunting by Sokka, who, after finding his fishline being woven by Aang, waded into the water to catch it barehanded. After a short struggle under water, the fish was caught. However, the se tu squirmed free and slapped the Water Tribe warrior against the head when he was distracted by mocking Aang and pretending to kiss the fish.
The se tu has two short whiskers positioned directly below its eyes, a long dorsal fin running from the top of its head down its spine, and two greenish-yellow pectoral fins positioned below the large blue stripes that traverse the sides of its body.
These fish are known for their habit of repeatedly jumping high out of the water, often looking at spectators as if they were taunting them. The fish are said to have "thoughtful" eyes and human-like features. They are the favorite food of the platypus bear and have been known to grow to the size of a full-grown man. Once a year, these fish swim hundreds of miles up river to spawn.
The se tu is similar to the catfish in appearance and fresh water habitat, although there are no solid green catfish like the one seen in the World of Avatar. The se tu is also similar to the salmon in their spawning ritual.