A being of immense power, Kukulkan comes from a race that was spacefaring and powerful before humanity ever discovered fire on Earth. Lonely, he traveled to Earth to teach the civilizations there peaceful ways, so they would not go the way of his own once-destructive civilization. Apparently, Kukulkan stayed for hundreds of Earth years, influencing the Mayan and Aztec cultures, as well as the Toltecs (who knew him as "Quetzalcoatl"), Egyptian, and even Chinese cultures. With an appearance that resembled a dragon (green snakelike face, yellow eyes, a single pink horn, blue feathers and wings, and a serpentine red and pink body), Kukulkan became known as the "winged serpent god," and he was respected for the knowledge he brought. When he left Earth, he promised to return when summoned, even giving the Mayans a remarkably accurate calendar. But they never summoned him, and Kukulkan traveled the spaceways, forgotten and alone, collecting peaceful specimens for his intergalactic zoo.

In 2270, Kukulkan sent a space probe to Earth, but he was angry to find that humanity was more brutal and warlike. When Captain Kirk and his crew aboard the U.S.S. Enterprise encounter Kukulkan (on stardate: 6063.4), he traps the vessel and tests the humans to see if they should be destroyed. Eventually, Kirk and the others convince him that his children are grown and seeking peace in their own ways, and he departs, feeling no less forgotten, but at least with the knowledge that his long-ago teachings may have helped humanity — and the universe — after all.

Kukulkan travels in a ship that is surrounded by an energy field that projects a holographic image of a winged serpent.