Bumi was the wild-eyed, eccentric, elderly King of Omashu. Prior to the start of the Hundred Year War, Bumi had been a close friend of Avatar Aang, and he remained the only known friend from Aang's childhood still alive by 100 AG. He became an old and wild man, prone to cackling and snorting laughter at jokes. Despite his apparent frailty, Bumi was an earthbending master, at one point claiming himself to be "the most powerful earthbender you'll ever see".

Bumi's eccentricity arose out of the fact that he always kept his mind "open to the possibilities". As a child, Bumi decided to use the Omashu delivery system as a chute ride, for which Aang dubbed him "a mad genius", a name that Bumi seemed to appreciate. Although Bumi appeared senile, he was in fact a brilliant and patient tactician, willing to wait until the opportune moment to strike in order to produce the most favorable outcome. Bumi carried himself with the same composure during a quiet, peaceful meal as he did when his city was under attack and he rarely explained his actions, which tended to infuriate his subordinates.