Janemba was the main villain from the 12th film, Fusion Reborn. He started as a giant, child-like blob. Similar to Majin Buu, he eventually turned into a twisted demonic fighter.
Janemba's teleportation ability and swordplay made him more than a challenge for both SSJ3 Goku and Vegeta.
In the right hands, this character could be a destructive force.