Who was the Avatar immediately before Aang?

The Avatar immediately before Aang was Roku. Roku was the Avatar of the Fire Nation and the previous incarnation in the Avatar cycle before Aang, an Air Nomad.

Roku was born in the Fire Nation and became a close childhood friend of Prince Sozin. After mastering the four elements, Roku served as a balancing force among the nations. His relationship with Sozin later deteriorated when Sozin pursued imperial expansion, while Roku insisted that the Avatar's duty was to preserve peace and balance.

Roku's life and death strongly shape Aang's story. Roku died while trying to stop a volcanic eruption, and Sozin abandoned him rather than risk delaying the Fire Nation's plans. After Roku's death, Aang was born as the next Avatar. Roku continued to guide Aang through the Spirit World and visions, including advice about the responsibilities and dangers of the Avatar's role.

The order is sometimes confused because several earlier Avatars appear in the series: Kyoshi, Kuruk, and Yangchen. All came before Roku, but none was immediately before Aang.