Bumi was the municipal King on an Earth Kingdom city called Omashu. Before the Hundred Year War, Bumi and Aang were good friends. Bumi grew into an earthbending master and a wild man. Many said Bumi was mentally unstable but despite this he had a sense of honor and was a man of his word. Unlike most "kings" or "leaders" Bumi was not born of royalty or even nobility, he earned his title through his heroic actions by chasing out a violent gang called the Hankering Cougars that took over the city.
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Bumi's Capture
The Fire Nation was sieged by a the Fire Nation including an Army that included Major Hakon and Colonel Yuzin. Bumi's forces were quickly overwhelmed by the larger and more advanced Fire Nation troops. After the battle Hakon and his unit were transferred out of the city while other forces stayed to occupy it. At some point, Ozai ordered a statue to be built in the city in his honor.
Aang's Failed Rescue
Avatar Aang attempted to free Bumi from captivity during a negotiated prisoner exchange between the Omashu Resistance and local Fire Nation forces. Bumi revealed that he was still capable of bending though being held by constraints but told Aang he intended to free himself when the time was right so he could analyze a weakness of the Fire Nation and strike from within.
Liberation of Omashu
During an eclipse, Bumi noticed the Firebenders were unable to use their power. He used that moment to break out of his constraints and defeat fire army troopers. Soon after, he destroyed the bridge that linked Omashu to other locations and tossed Ozai's statue off of the city.
Bumi or Toph?
Many argued that Bumi was the most powerful earthbender of his era while others said Toph was the greatest of her time. Though Iroh later concluded that Bumi was the greatest of the Hundred Year War generation while Toph was the best earthbender of the next generation.