Gotenks is a fictional character from Dragonball Z. He is the result of the fusion dance between young Goten and Trunks. He appears during the Majin Buu Saga in an attempt to stop Majin Buu. It take the two Saiyans a couple of attempts to pull off the fusion, since their movements must be exact. Gotenks is also the only character to appear in the series through the fusion dance, and is the second (and final) character to become a Super Saiyan 3.
 Plot Involvement
Realizing that Buu was on the verge of destroying Earth, Goku decided to teach Trunks and Goten the fusion dance, in an attempt to merge into one powerful being, capable of saving everyone. After learning the technique, the two youngsters failed twice during the dance, creating a "fat" Gotenks and a "skinny" Gotenks, both with very weak power levels. Once the fusion dance was perfected though, Gotenks was born.
Gotenks is a very cocky and arrogant fighter, often times not taking his opponents and fights very seriously. He believes that he is the strongest fighter in the universe, and has no reservations about letting others know. He creates many of his own techniques, but only a few are anything but comical and weak. After the initial transformation, Gotenks goes to fight Majin Buu but is easily defeated.
After upgrading themselves to Super Saiyan, Gotenks, along with Piccolo, lure Buu into the Hyperbolic Time Chamber. Once inside, the two fight once more, with Gotenks feigning weakness in an attempt to make the battle more dramatic. Piccolo, fearing trouble, destroys the exit so they are trapped. Buu is able to tear a hole back to the other dimension through a loud scream.
In order to escape, Gotenks transforms into a Super Saiyan 3, producing a scream that allows them an exit as well. Once outside, Gotenks goes after Majin Buu again, damaging him heavily with his Super Ghost Kamikaze Attack. Before he could attempt to finish off Buu, the fusion dance wears off and Goten and Trunks are individual beings once more.
Later on, while fighting Gohan, Super Buu declares that he wishes to fight Gotenks again. After the children fuse, Super Buu captures them (along with Piccolo) into his body, drastically increasing his strength.
Base Form - Gotenks is small in stature, much similar in height to that of Goten and Trunks. He gains quite a bit of muscle mass though, while sporting a yellow and purple vest and white pants. His hair is dark black, with purple streaks running along the sides. In this capacity, Gotenks is not very powerful, but still arrogant as ever. He believes he can stand up against Majin Buu but ends up being defeated quite easily.