The Hyper Demon is a fan fiction character of the Dragon Ball Mulitverse. He was born shortly after the defeat of Janemba at the hands of Gogeta. He is the merger of all evil souls that pass through the soul cleansing machine and would be sent to hell if they were not apart of him.
The Hyper Demon looks just like the Saike Demon in Movie 12 Fusion Reborn, as the Hyper Deomon is basically the Saike Demon but an adult, taller, muscular and more mature version. In his suppressed form his height is similar to Zen Buu, but in his 100% form his size is similar to the tallest characters in the Dragon Ball Multiverse alongside Broly, King Cold and Gast Carcolh.
Originaly the Saike Demon worked for King Yemma in the Spirit Laundry in Hell, but the Demon became dsitracted listening to heavy metal allowing the Soul Cleansing to overflow with eveil spirtis mutating the Saike Demon's body and creating Janemba, a demon of enormous power. Proving to be too much of a match for even Super Saiyan 3 Goku and a Super Saiayn 2 Vegeta. He was eventually defeated by their fusion Gogeta.
Shortly after the Saike Demon ran in fear from the Super Saiyan he felt bad about the imbalance and destruction he caused the universe and wanted to use the power he had had as Janemba for good, so he went off in search of the Old Kai and Kabito Kai hoping that they would train him to be mentally strong enough to control the evil of Janemba. They did so through the art of meditation and pshycic training after two years of mental training the Saike Demon felt he was ready and reinacted the same accident that had created Janemba two years ago. Only this time the dimensions were not affected nor did his apperance become monstorous because he was able to control the conglomeration of evil spirtis that was Janmeba. Instead he appeared to become an adult much taller, muscular, more mature and the same size as Janemba's final form. He used this newfound power for good by easily defeating a Super Perfect Cell, Frieza, his father and the Ginyu Force who were creating chaos in Hell trying to get back to the living world.
However the Saike Demon remembered the power of Gogeta and how even the power of Janemba was no match for it. He began to ponder what if a villan of that power attacked the Other world. He asked of King Yemma's permission to abosrob all the evil souls in the Soul Cleansing machine, that came to hell and that already inhabited it. Having already shown his attonment and being a completely new being from the young childish Saike Demon he was King Yemma agreed. The Saike Demon again absorbed all the evil spirits from the soul cleansing machine and all of Hell's inhabitants under orders of King Yemma and all evil spirits that were to be sent to Hell would be absorbed by the Saike Demon allowing him to transform into the Hyper Demon and become the living embodiment of Hell. He would later go on to defeat Broly who was attacking the Other world after death and Majin Buu who Babbidi released on the planet of the Kai's.
The Hyper Demon was already very strong when he was the Saike Demon reunited with Janemba but when he absorbed all the evil souls of Hell and the Soul Cleansing machine his power skyrocketed. In his fully supressed form the Hyper Demons strength and size is the same as it was when the Saike Demon first reunited with Janemba or final form Janemba himself. However when in his 100% full power form his power skyrockets and so does his height and muslce mass his size surpasses that of King Cold and is similar to Broly's. In this form the Hyper Demons power far surpasses any participant in the multiverse tournament combined. With a power level 400,000,000,000,000,000,000 his power far exceeds even Super Saiyan 3 Vegito's. During his fight with 16 The Hyper Demon does not move an inch, probably to keep from showing his ki but as he leaves the ring Vegito still senses the hidden energy and says he'd never felt anything like it and it was as if a whole dimension was inside of him. Bra asks why he's worried having never seen her father take anything as a threat, making the remarks that Vegito was calm about the idea of facing Zen Buu and Broly who where the most dangerous and strongest competitors in the tournament respectively. Vegito says that the power of the demon surpasses both of them. The Old Kai also mentions that the Demon because all of Hell's populous and all evil spirits that would be sent there are inside of him it causes him to become very sadistic at times, this along with his ability to warp reality, control space and time and transmutate matter make him the most unpredictable contestant in the tournament as he is normally good.
The Hyper Demon preffers not to use his 100% full power form unless he absolutely has to because this form taps into the full power of the mass of evil spirits the Saike Demon absorbed, because of this even with his mental training he can't control the form for long or the evil will overtake him causing him to go on rampages of destruction attacking anyone and destroying planets, Galaxies and evem entire universes in his wake. However the Hyper Demon is still very committed to his meditation and mental training and strives to one day be able to control the power of his final form without letting the agglomeration of evil spirits overtake him.
The full scope of the Hyper Demon's abilites are unknown for he has only appeared for one fight so far; against Android 16. The andoid assaulted him with a Barrage of punches and kicks but had no effect the Demon just smiled at him grabbing his leg as he prepared for another kick, transmutating it into cheese. The android struggles to maintain his balance on one leg but eventually gains control and flies into the air releasing a powerful Hell's Flash attack. When the smoke clears the Hyper Demon is completely unscaved. The android makes one last attempt to defeat the demon by flying at him head on preparing for a powerful punch to the Demon's stomach but having not moved since their match began and still continuing to smile and laugh under his breathe at the android attempts. Part of his stomach morphs into a dimensional portal and Android 16 falls through, it was too late for him to escape as his punch was about to connect with the demon's stomach though it wouldn't have done any damage anyways. He then closed the portal and sealed the android leaving the ring with a smile as if nothing had happened, showing that sometimes even in his suppressed state the massive amounts of evil spirits present within him may get the better of him. Vegito caught a glimpse of the Hyper Demon's true power when his stomach briefely morphed into a dimensional hole to hell and it was enough to make even the mighty fused super saiyan fear him.
It is clear that the Hyper Demon posses all of Janemba's abilites and more as he has absorbed all of the souls that once populated hell or it's Soul Cleansing machine, it is implied that the masive amounts of evil spirit energy he has absorbed have augmented the strong mental abilites the Saike Demon gained through meditation and mental training giving him massive psionic, telekinetic, psychic abilites as well as augmenting his reality warping abilites. As the only time we have seen him is his match with Android 16 his full abilites haven't been shown.