North Kaio's World
From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
North Kaio's World (北 の 界王星, Kita no Kaiō-sei; FUNimation "King Kai's Planet"; Literally meaning "Northern World King Planet") is a planet in the Kaio Realm.
North Kaio's World was home to North Kaio, ruler of the North Galaxy. It was also home to North Kaio's pet, Bubbles, and attendant, Gregory. Even though the planet was relatively small, it had a gravity 10 times that of Planet Earth, allowing for more intense training (clearly seen through Son Goku's struggle to even stand up).
After Goku sacrificed himself to kill Raditz, he was taken to the Check-In Station by Kami. Kami asked Great King Yama to allow Goku to travel down the Serpentine Road in order to train with the legendary North Kaio in preparation for the upcoming battle against evil Saiyans when they arrived on Planet Earth in one year. Yama granted this and Goku spent many months heading towards the planet. Upon arrival, Goku's skills were put to the test. He was assigned to catch Bubbles (which proved a daunting task considering the gravity) and to hit Gregory with a hammer (which was fairly hard to hold let alone swing). He passed these tests and was permitted to receive North Kaio's training. During the next few months, Goku mastered the Kaio-ken and the Genki Dama, techniques that would be essential in his battle against the Saiyans. Goku later visited the planet during the ten-day wait to ask for directions to New Planet Namek, and also being summoned by North Kaio in order to deal with a Super Saiyan that had destroyed the South Galaxy.
The next time North Kaio's World was actually visited in the main story was when Goku teleported Cell to it as he was about to Self-destruct. This killed North Kaio, Bubbles, Gregory, and even Goku. North Kaio, his pets, and Goku now had to live on Grand Kaio's World until North Kaio's World was re-created.
- Son Goku - Visited numerous times throughout the series, but mainly to receive training to battle against the Saiyans.
- Piccolo - Killed by Nappa after protecting Gohan from the giant Saiyan's energy blast.
- Tenshinhan - Died after giving off all his energy in an attempt to kill Nappa.
- Yamucha - Killed by a kamikaze Saibaiman.
- Chaozu - Blew himself up in an attempt to kill Nappa.
- Olibu and various other North Galaxy heroes. All died under unknown circumstances (although the only one heard of was Olibu).
- Great King Yama - Traveled to receive training.
- Ginyu Force (Barring Ginyu) - Killed on Planet Namek by Vegeta. Were given a special invitation by North Kaio in order that his students (Yamucha, Chaozu, and Tenshinhan) could be assured that their training was helping them.
- Cell - Forced there after Goku used Instant Transmission in order to save Planet Earth from Cell's self destruction.