Roku

Roku was the Fire Nation-born Avatar immediately succeeding Avatar Kyoshi and preceding Avatar Aang. Roku's memory served as a guiding force for Aang throughout his quest to end the Hundred Year War.
Roku was a wise and merciful Avatar with good intentions, though he himself acknowledged that he was too indecisive during his life. Roku's long friendship with Fire Lord Sozin resulted in his reluctance to end his friend's life and thus his imperial ambitions. This ultimately led to Roku's death, preventing him from stopping a war which would last for one hundred years. Aang inherited Roku's burden of healing the war-torn world, and Roku firmly believed Aang was destined to amend his mistakes.