Mukuro Rokudo is the first antagonist in Katekyo Hitman Reborn but eventually takes on a semi-ally roll.
Rokudo Mukoro was raised being experimented on along with several other children. It is here that he gained his eye that gave him his powers. However he was able to eventually use these powers to kill his captors and escape along with Ken Joshima and Chikusa Kakimoto. Together they formed their own "Family" and head out into the world to take over the Mafia. During this time Mukuro gathered together several other members to his family to help him with this goal, including Lancia, a man who he took control of with his mind controlling powers and forced him to kill his entire family.
Mukuro traveled to Japan with his subordinates and enrolled in Kokuyo Junior High where he created a gang that he used to declare war on Tsuna's school in an attempt to draw him out. Tsuna had no choice but to go to Kokuyo Land, an abandoned amusement park, where he met Mukuro, who was pretending to be a normal student who got lost there. However after Tsuna ran away he trapped Tsuna in a theater and revealed who he really was. Hibari stepped in to fight Mukuro but his body was already too beat up to fight very well and he was defeated. Mukuro used one of his powers Six Paths of Reincarnation powers to possess the bodies of Tsuna's defeated friends, but when Tsuna doesn't run from them he receives new powers from Reborn and Leon that allow him to save his friends and defeat Mukuro. After the battle the Vendice show up to arrest Mukuro and take him to one of their secret prisons that are inescapable.