The aye-aye spirit is the caretaker of a Spirit Oasis within the Spirit Wilds. Although he once harbored an intense prejudice toward humans, the spirit developed a bond with Wan which allowed him to see the human potential of nobility and selflessness. He even gave Wan the name of "Stinky", as he jokingly stated that he viewed Wan as his pet.
After being banished by the Chou family into the Spirit Wilds, Wan stumbled upon the Spirit Oasis which the aye-aye spirit guarded and noticed several trees bearing fruit on the island. Hungry and in need of a place to rest, Wan made his way toward the oasis, though quickly found his path blocked by the spirit, who appeared out of nowhere and denied him access. Wan attempted to reason with the spirit, but to no avail, and resolved to using firebending in an attempt to ward him off. This act only enraged the aye-aye spirit, who teleported to grab Wan from behind and threw him aside, reasserting his decision.
Shortly after, the aye-aye spirit began granting access to a parade of spirits and caught Wan disguised as a bush spirit, recognizing his distinct human scent. He ordered Wan to leave once more, refusing to give him directions to the other lion turtle cities. The aye-aye spirit witnessed Wan leave, and later saw him fend off a group of hunters in an attempt to protect a captured cat deer. After Wan had been cornered, the spirit possessed one of the hunters, prompting the other to flee in fear. He carried Wan to the oasis, where the spirit waters healed him of his injuries. When the other spirits agreed with Wan's decision to stay in the wilds to learn the ways of the spirits, the aye-aye spirit initially disagreed, but eventually relented.
Over the course of several years, the aye-aye spirit developed a close friendship with Wan, and was present to watch Wan master his firebending skills by learning the Dancing Dragon form from a white dragon. Together, the two of them protected the oasis from trespassers until Wan's departure to find the other lion turtle cities. Before Wan left, the aye-aye spirit told the young man that he was proud to call him his friend.
Years later, a gathering of spirits led by the aye-aye spirit came into conflict with Jaya and other citizens from Wan's old city. When Wan intervened, the aye-aye spirit was happy to see him, believing he had arrived just in time to drive away the settlers. Though Wan attempted to mediate the situation, his efforts proved to be futile. The negative energy from the standstill allowed Vaatu to turn the aye-aye spirit and the other spirits dark. As the firebenders provoked the dark spirits with a single fireball, the aye-aye spirit ordered his fellow spirits to attack, but both sides were kept at bay by the efforts of Wan and Raava. The aye-aye spirit briefly reverted to normal when Wan merged with Raava, creating a positive-energetic barrier of all four elements between the two groups. However, as Wan was being carried away after caving under the strain Raava's presence put on him, the aye-aye spirit again succumbed to Vaatu's dark energy and attacked, slaughtering all the settlers with his fellow spirits.
After Wan defeated Vaatu during the second known Harmonic Convergence, the aye-aye spirit and the other spirits who roamed the wilds returned to the Spirit World. Before parting ways, the spirit respectfully bowed to the Avatar.
The aye-aye spirit is spiteful of humans, believing that they are all "ugly, destructive, and lacking any respect for nature". Prior to befriending Wan, he was highly condescending, and only eased on his actions after recognizing Wan's nobility and selflessness. Regardless, he considered Wan as an exception, and remains largely hostile against other humans.
As a spirit, the aye-aye is capable of teleporting over short distances and possessed much more agility and strength than an average human, having displayed the ability to overpower Wan in combat. He also has the ability of possession, allowing him to temporarily take hold of a human body and control it as if the body were his own. This process caused a transformation in the human host, who was permanently left with some of the spirit's physical attributes. The aye-aye spirit can potentially kill humans by remaining inside their bodies for a prolonged period of time.