Como explicado no filme (e citando aqui o "Villains Wikia" ):
Roman is the husband of Marianne Armitage, the father of Dean Armitage, the father-in-law of Missy Armitage, and the grandfather of Rose and Jeremy Armitage. According to Dean, Roman participated in the 1936 Olympics, but lost to the famous Jesse Owens.
Unable to take his loss seriously, Roman pathetically developed a hatred towards (and obsession with) black people and founded the Order of the Coagula to perfect a process - kidnapping black people in order to brainwash them and to implant the brains of old relatives and friends into the bodies of the far younger and fitter black people. As Roman and Marianne grew old, Dean (taking over as the new leader of the Order) used the family's groundskeeper Walter and housekeeper Georgina as new vessels for the dying Roman and Marianne. Ever since then, Roman used Walter's body to become more athletic and stronger until the film's events.