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. Its actions were perceived as taunting by Sokka, who, after finding his fishline woven into a necklace by Aang, waded into the water to catch it barehanded. After a short struggle underwater, the fish was caught. When Sokka attempted to kiss the fish while mocking Aang, the se tu increased its struggles to squirm free and slapped the Water Tribe warrior against the head with its tail before plunging back into the water.
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.
- These fish are similar to the "fish-men" from The Legend of Zelda: The Wind Waker.