He is the Lord of Demons and in charge of the Underworld. At the beginning of the game, Garcia Hotspur, the main protagonist had just defeated a demon but not until he warns Garcia about demons getting ready to attack his girlfriend, Paula. Garcia makes his way to their home and sees her hung, having committed suicide. Garcia kills several demons until meeting Fleming. Garcia shoots Fleming several times with Johnson's gun form but is unhurt as he takes Paula. Garcia then chases him to the Underworld.
Garcia sees Fleming's castle, which the demons call the "Castle of Hassle" due to it having a very hard to pronounce name, and walks the City of the Damned to get to it. Along the way, he kills and defeats several of Fleming's "V.I.P.s," demons granted special abilities by Fleming, including his three adopted daughters, who have the ability to kill demons permanently and resembled Grim Reapers. Killing the V.I.P.s dropped blue gems, which helped make Johnson much more powerful and Garcia and Johnson make it to the Castle of Hassle. Along the way, Johnson tells Garcia the story of the Unbreakable Huntress, a female demon hunter who tried to defeat Fleming but had her limbs severed. Fleming spared her life and made her his queen, seeing her strong will, and killed her over and over again. It is heavily implied in the game Paula was the Unbreakable Huntress. Garcia makes it to the top of the castle and after a long battle, seemingly kills Fleming. Paula attacks Garcia, finally going mad from all the torture she suffered and becomes an angelic demon. Garcia defeats her as the final boss of the game and calms her down. At the end of the game, Garcia gets a call from Fleming, alive and well and says he's going to send more demons after him to get Paula back. Garcia says he will kill every last one of them while Paula ends up in a trance and he aims his gun at several demons, hinting at a sequel.
Fleming is a very powerful demon having complete control over the Underworld. He sometimes takes pity on some souls and makes them his V.I.P.s, which act as the bosses of the game. Johnson tells Garcia that even other demons don't like Fleming but as he was "top dog," the demons just go with the flow. Aside from taking Paula away from him, he also despised Garcia for killing many demons.