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Fusion Characters
The trick of merging two people together to form a new and much more powerful being is known as Fusion-waza. Even though the resulting entity is more powerful than the combination of its separate components, it is able to remain intact for only 30 minutes. After 30 minutes, the fusion person separates back into the two people that formed him. It was Son Goku that learned the technique of Fusion-waza while sojourning in Heaven.
Goku taught it to Trunks and Son Goten during the Buu Saga. Trunks and Goten merged several times before getting the dance-like move down properly. Together, they formed Gotenks.
Gotenks has got to be one of the most obnoxious characters of the Dragon Ball world. He has got an ego worse than Vegita, a power level that might rival Goku's and a childish behavior fit for the brat of brats.
Later, Goku and Vegita used Kaiou-shin's earrings to merge into the most powerful being in the Dragon Ball universe, Vegit. He was one bad dude. Vegita has vowed to never fuse with Goku, again so Vegit will probably never be seen again.
Fusion persons tend to have the worst traits of the people merging to form them.