Mr. Popo is an immortal djinn who lives with Kami in Kami's Lookout and is bound there eternally as the gardener and caretaker. He has served every other Kami (God/Guardian) of the Earth since the beginning of time. He was trained by the recent Kami himself and trained Goku for 3 years in order to fight Piccolo.
Mr. Popo is an excellent martial arts master. When he is introduced in Dragon Ball, his fighting prowess is far superior to Goku's in nearly all respects, when the two meet at the Lookout.
Mr. Popo and Kami train Krillin, Tien Shinhan, Yamcha, Chiaotzu and Yajirobe in order to help combat the approaching threat of the Saiyans, through many different training methods, including the Pendulum Room, intending to show the full power even the lower class Saiyans used to possess. Mr. Popo and Kami come to the realization that Piccolo has become far less evil than he was five years ago, when they notice that Raditz's soul, who was killed by Piccolo, did not become lost like other souls killed by demons.
Mr. Popo appears again in the Imperfect Cell Saga, bidding Kami a final farewell when Kami permanently fuses with Piccolo to become the Nameless Namek, in order to gain more power to combat the Androids who are wishing to murder Goku.
When Buu escapes from the Hyperbolic Time Chamber, Mr. Popo and the rest are turned into chocolate and eaten. However, before he was turned into chocolate, after seeing the murders of Android 18 and her daughter, Popo bravely makes a quick decision, seizing Dende and hurling him off of the lookout, out of Buu's rampage. He is commended for his bravery by Piccolo, when the Namek comes across the surviving Dende.