Grubba is a boss character who just so happens to be in possession of one of the Crystal Stars in the video game Paper Mario: The Thousand Year Door. He is the true main antagonist of Chapter 3.
Grubba ran a wrestling pit known as the "Glitz Pit" in a fancy town floating above the clouds called Glitzville that accommodated fighters from all over. The middle of the arena held one his most horrific secrets, however, as it was that Grubba was actually stealing the souls of his top fighters. No one seemed to suspect him except for his assistant, Jolene, an elder sister Toad of one of the original champs named Prince Mush. When Mario found out about Grubba's horrendous secret, Grubba brought out his soul-stealing machine which happened to have a Crystal Star as its fuel source. He revealed that the machine was being used to keep him looking young forever. Prince Mush had been one of his first victims it turned out. He then used the machine to become Macho Grubba, a powerful version of Grubba.
Macho Grubba is an ultra-charged version of Grubba, formed through Grubba's soul-stealing machine. He is the final boss of Chapter 3 and is capable of reviving himself several times during battle. Mario and friends eventually managed to take him down and the original champion, Prince Mush, emerged from the machine and reunited with his sister (and Grubba's former secretary), Jolene. Mario was then given the third Crystal Star by Jolene, who later took over the Glitz Pit in place of Grubba. It is unknown what happened to Grubba afterwards.