Janemba is the main villain from the 12th Dragon Ball Z movie; Fusion Reborn. Janemba was created when a young ogre, who was put in charge of cleaning the Soul Cleanser, neglects the machine. This caused all of the negative energy from the machine to explode and transforms the ogre into Janemba. Goku is sent down to Hell to fight Janemba.
After a short fight Janemba made Goku go Super Saiyan 3 before he could defeat him, and when Goku did Janemba transformed into a far more powerful form. Vegeta in his Super Saiyan 2 state then appears, and joins the fight, but even with his help they can't defeat Janemba. Goku suggests that they fuse together, and Vegeta agrees. They transform, but not correctly instead forming into Veku, so Janemba beats him around until the fusion wears off. They try again and do it right forming Gogeta who beats Janemba in no time with the Stardust Breaker, dissolving the evil that is Janemba and turning him back into the ogre .
His first form actually seems to be called Janempa, as it's all he says, and is referred to as such by Goku and Pikkon, as well as the credits when voiced by Jim Foronda, while his second form is referred to in the credits as Janemba, voiced by Kent Williams. However, the games call him only Janemba, and his second form is referred to as Super Janemba.
Like many Dragon Ball villains, Janemba is a destructive and aggressive monster who seems to have little motivation other than sadistic pleasure and lust for anarchy. In his first form, he appears more childlike and innocent, but becomes much less so in his second one.
He's capable of using magic to alter the environment to his liking, with little regard for how it affects others, showing an obsessive need to control. If someone insults his magic it becomes slightly weaker for a while, proving that his powers are deeply connected with his emotional state and that he's easily offended.
The evil energy which eventually manifested into Janemba had been gathering for generations, and created a powerful evil heart. The evil energy that he was born from includes that of many humans. In the GBA game Dragon Ball Z: Buu's Fury, King Yemma tells Goku that Janemba was the kingpin of an organized crime syndicate in the Demon World before he came to Hell. His scouter profile in the game also states this, saying that the soul of the crime boss Janemba had just arrived in the soul cleansing machine, and turned into a monster after taking over the Saike Demon.
Janemba is incarnated in the Other World into the Saike Demon, a teenage ogre responsible for watching the Soul Cleansing Machine, when the latter slacks off and causes the cleanser to explode. The evil formed around the ogre mutates him into a giant, yellow, and childlike being who begins to turn Hell into his own paradise.
Goku and Pikkon are interrupted from their match in the Other World Tournament to fight Janemba, Pikkon being left behind to free King Yemma from the barrier that Janemba created around the Check-In Station.
Eventually, Janemba's weaker first form is destroyed by Super Saiyan 3 Goku, forcing a transformation into his second state, which enables him to overcome Goku. Vegeta appears and assists Goku, but neither of them is strong enough to defeat Super Janemba. The two eventually defeat Super Janemba by fusing into Gogeta, who destroys Super Janemba by purifying his pure evil energy and turning him back into the Saike Demon.
Powers and Abilities
Upon sensing the power of Janemba in his first form, Goku says that it is the strongest power he has ever felt (he may have been referring to Janemba's reality manipulation abilities along with his raw strength). In his super form, Janemba manages to completely overpower Super Saiyan 3 Goku and is only defeated by Super Gogeta (the fusion of Goku and Vegeta, whose Power Level is said to be 2,500,000,000 in a 2004 V-Jump). According to the Toei website for the Dragon Ball/Dragon Ball Z movies, Super Janemba is stronger than even the Genmajin Hirudegarn, since Hirudegarn was capable of trading blows evenly with Super Saiyan 3 Goku while Super Janemba could beat back Super Saiyan 3 Goku. Super Saiyan 3 Goku seemed to be more powerful than Ultimate Gohan when fighting Hirudegarn, yet was no match for Super Janemba.
His power over dimensions is capable of transforming the entire universe into his image, including the Other World. He possesses great matter manipulation powers as well since he was easily able to pick up a club and alter its molecular structure, turning it into a sword. Aside from his power over dimensions and matter, he is also proved to be an accomplished martial artist, taking on both Goku and Vegeta in hand-to-hand combat, and winning. In both forms, Janemba has a rather unusual weakness: he is physically hurt by insults. Whenever he or any of his constructions are insulted, he or it cracks slightly and shatters like glass. However, it is not enough to be useful against Janemba due to his powers of rapid regeneration, and stuns him only briefly. His constructs, such as the barrier around the Other World station, do not regenerate, but, like Janemba, crack only slightly and must be insulted numerous times before they can be broken. Only Pikkon seems to be aware of this weakness, as he got frustrated and insulted one of Janemba's creations.
The story of the PSP game Dragon Ball Z: Shin Budokai has it that Janemba's power level is based on the amount of evil energy he has absorbed, as the likes of Pikkon and Super Saiyan Goku can hold their own against Super Janemba at the start of the game, but by the end Janemba is able to fight an almost even match with Gogeta/Vegito and is able to take down Kid Buu. Upon Janemba gaining physical form, Kibito Kai states that he feels like he is being pulled apart just by looking at Janemba. When Gotenks fights Super Janemba, he states that he does not seem to be doing any damage even using his full power as a Super Saiyan 3. In the arcade mode of Shin Budokai, after meeting Super Janemba, Super Buu (Gohan absorbed) states that he is stronger than Janemba.