Gotenks is the result of the Fusion Dance when performed by Son Goten and Trunks. Gotenks is the only character in the original series that was produced through the fusion dance. When Gotenks was made in the original series, it took several attempts for Goten and Trunks to successfully produce him. Gotenks is seen to be cocky and arrogant because he has trunks personality, as well as having the ability to be easily distracted. At the start of his appearance, Gotenks is rebellious, however, he later learns to follow the advice of his mentor Piccolo.
During his training, he reveals the ability to transform into a Super Saiyan 3, where he nearly defeats the villain Buu. The immense ki required for Super Saiyan 3 limits the time that Gotenks can stay fused, and he is later absorbed by Buu. After defusing within Buu's body, Gotenks is not seen in DragonBall Z again except for a filler episode near the end of Dragon Ball although he does appear in several movies notably Yo Son Goku and his Friends Return! where he is made after Abo and Kado (the movies villains) fuse and Battle of the Gods where he attempts to fight Bills when Bills attacks the Z Fighters although he is quickly defeated and spanked after which he falls to the ground crying.
When Son Goten and Trunks tried the fusion for the first time, the entire fusion dance was perfect until Trunks extended his fingers instead of using his fists. They fused into Fat Gotenks. At first, everyone thought he was supposed to fuse like that, that that kind of body must help to regulate the enormous power at that level. But they later find out the fusion had failed. After half an hour they tried it again and at the end of the fusion dance they both used their fingers. Normally the tips of their fingers are supposed to connect, but the fingers bumped into each other instead. So this time they fused into Skinny Gotenks. He looked old, even older than Master Roshi. They had to wait half an hour again. Finally, the third time, they made it. But Gotenks seemed to be very cocky and arrogant in contrast with trunks attitude. Piccolo said they should wait for another half hour but Gotenks thought 'they' were strong enough as it is. Gotenks thought he could beat Majin Buu but he was very wrong. Later on, they tried fusing as super sayians. It shows that the super saiyan Gotenks is even cockier. They have no other choice so Gotenks does the same thing Goku showed them while he was visiting Earth. They look deep inside and find the hidden power to turn into the amazing super saiyan 3! With his enormous power he goes after Buu, yet he still doesn't have enough power to defeat him.
Super Saiyan - This form can be reached either by Goten and Trunks performing the Fusion Dance correctly as Super Saiyans or Gotenks can transform into this state whilst fused. This form is approximately 50 times stronger than Gotenks' ordinary state yet was not enough to match up to Super Buu
Super Saiyan 3 - This transformation was attained whilst Goten and Trunks trained in the Hyperbolic Time Chamber in preparation for facing Majiin Buu. Having remembered what Goku had told them about transforming into a Super Saiyan 3, Gotenks proved able to reach this form. Gotenks' power proved sufficient enough to tear a hole through dimensions to escape from the Hyperbolic Time Chamber. Also, SSJ3 Gotenks was at least equal in power to Super Buu, if not slightly stronger.
Big Tree Cannon - A move obtained from Trunks, the attack is similar to Vegeta's Final Flash in that the user gathers a large amount of energy in both hands then brings them together in a gun like shape, firing the energy at the opponent.
Galactic Donut - Gotenks creates an energy halo that constricts his oppoenets within the halo. Gotenks then causes the halo to tighten around the foe.
Super Ghost Kamikaze Attack - Gotenks' signature technique. Gotenks spits out 'Kamikaze Ghosts' from his mouth and commands them to fly towards an opponent. The ghosts explode on contact with an object or person.
Charging Ultra Buu Volleyball - After trapping Super Buu in many Galactic Donuts and condensing them into a ball, Gotenks then used the coccoon as a volleyball, passing it to Piccolo before slamming it to the ground.
Die Die! Missile Barrage - A rapid fire energy attack where Gotenks fires a barrage of Ki blasts at the opponent.
Revenge Cannon - A beam of energy that is fired from Gotenks' mouth.
Vice Shout - A shout capable of breaking the barrier between dimensions.