Gotenks
A character formed by Goten and Trunks merging through Fusion
History: In order to defeat Majin Buu, who boasted absolute strength, Goku taught Fusion to Goten and Trunks as a last resort, and thus Gotenks was born. After several failures, they finally succeeded in merging together. The two entered the Room of Spirit and Time, and hurriedly trained as Gotenks.
As a result, Gotenks leveled up so much that his strength surpassed Vegeta and the others. However, they were taken in by Buu, who had powered up by absorbing the good portion of himself.
First Appearance: chapter 480
Special Characteristics: He has Goten’s hair color and Trunks’ feature. He has an extremely haughty personality, as if only the bad parts of Goten and Trunks’ personalities show through. There is a normal-type Gotenks as well as a Super Saiyan one, and he can also transform post-Fusion, being the only person besides Goku who can become a Super Saiyan 3. (Daizenshuu 2, p.118-119)
Techniques :Rapid-Fire Die-Die Missiles, Super Ghost Kamikaze Attack, Rapid-Fire Super Doughnuts, Super Ghost Kamikaze Attack—10-Ghost Finish, etc.
Source:
https://www.kanzenshuu.com/forum/viewtopic.php?t=32226