King Kai

King Kai lives at the end of Snake Way in Otherworld. Upon his death, Goku travels down Snake Way to train with King Kai. Before he would train him, Goku had to pass three tests. One was for Goku to tell a joke and make him laugh. So its safe to say King Kai has a sense of humor. King Kai's planet has 10x the gravity that Earth has, so training was hard at first for Goku. King Kai teaches Goku the Kaioken technique and the Spirit Bomb (Genki-Dama). Which enabled Goku to defeat Vegeta and others.

King Kai has also trained Yamcha, Tien, Piccolo, and Chao-Zu. King Kai died in the Cell saga, when Goku instant transmitted to his planet with an exploding Cell that blew up his planet killing him and Goku, Cell survived.

King Kai has the ability to see anywhere and talk telepathically to anybody in the universe. That ability came in handy in the defeat of Freeza and Majin Buu. During the Freeza and Goku fight he talked to Kami on Earth and told him to gather the Dragon Balls and wish everybody Freeza and his men killed on Namek back to life. In doing so, the Dragon Balls of Namek came back. King Kai then communicated to Namek and told them to use their Dragon Balls to wish everybody except Goku and Freeza off a slowly exploding Namek and onto Earth.

During the Goku and Majin Buu fight he communicated with the new planet Namek and told them to gather their Dragon Balls to wish back Earth, which Buu blew up, and all its people on it. Doing this Earth's Dragon Balls were also brought back. King Kai's abilty allowed Vegeta and Hercule to talk to the people of the universe and ask them to lend Goku some of their energy for the Spirit Bomb. King Kai then contacted Earth and told Kami to use its Dragon Balls to restore Goku's depleted energy. Which enabled Goku to defeat Majin Buu.