Hei Bai, also known as the Black and White Spirit, was a forest spirit that normally took the form of a docile panda bear but could transform into a large four-armed monster in the Spirit World. He has protected an Earth Kingdomforest for thousands of years.
Hei Bai was a spirit who protected a once-lush forest outside Senlin Village. Upon seeing his forest destroyed, Hei Bai became enraged. Unaware that it was the Fire Nation who destroyed the forest, he targeted the nearby Senlin Village and blamed the villagers. As the winter solstice drew near, Hei Bai transformed into his monstrous form and terrorized the village by abducting one villager each day. When Aang came to the village, he attempted to reason with the enraged spirit but to no avail. Hei Bai became violent toward Aang, prompting Sokka to help Aang by attacking Hei Bai with his boomerang. However, Hei Bai quickly overpowered and kidnapped Sokka and took him to the Spirit World. Later, in an attempt to calm the angered spirit, Aang showed Hei Bai the acorns that covered the floor of the burned forest, reassuring the spirit that the forest would grow back. Hei Bai's anger faded and he returned to his normal form. He took the acorn in his mouth and then left through the village's gate. From that gate sprouted bamboo stalks from which the missing villagers stumbled out.
After Aang met with Koh, Hei Bai returned to give him a ride back to the gateway out of the Spirit World. He shot a stream of light from his mouth toward Aang, sending him back to the physical world. In response to a rude comment from an odd spirit upon Aang's departure, he transformed into his monstrous form and blasted the disrespectful monkey, scaring him away, before changing back to his usual form.
Months later, while Aang was in a coma induced by PrincessAzula's lightning, Hei Bai appeared in his dream to help him in his quest to heal the Avatar Spirit by visiting his most recent past lives.
Hei Bai was very docile and friendly in his panda form; when angered, however, he changed into a six-legged, black-and-white monster and destroyed everything in his path. Somewhat vengeful and very protective of his forest, Hei Bai generally kept to himself and did not bother others unless they provoked him first. He was a powerful spirit with amazing strength, capable of decimating a structure with one swing of his arm. He could also move at great speeds, as shown when he was destroying one house and in a streak of light suddenly appeared in front of another building. He was able to use powerful sonic screams, the power ranging from a gentle breeze to a blast capable of leveling a building in seconds. He possessed the ability to carry individuals to and from the Spirit World. Hei Bai was also able to spontaneously grow bamboo.