One hundred years ago, young Bumi was friends with Aang and they use to play together in Bumi's hometown, Omashu. The two troublemakers often went down the delivery chutes, which were basically like giant super slides. They had a lot of fun, and Aang often called Bumi a Mad Genius because of his interesting way of viewing things. It was Bumi's motto to "open your brain to the possibilites" which basically meant not to accept just the obvious.
That's still Bumi's motto today, although now he's called King Bumi. He uses a lot of bad puns, and while it might seem that he's lost a few marbles, he is quite wise. And despite the fact that he must be at least 109-years old, King Bumi is in very good shape and is quite skilled in Earthbending. He also snorts when he laughs, has a large pet named Flopsie, and has a... unique sense of style.