The Chainsaw Majini was one of the most powerful type of Majini which appeared during the Las Plagas outbreak in Kijuju. Very similar to the Chainsaw Man encountered by Leon S. Kennedy, they were extremely resistant to damage, and were considered successful adoptions of the Plaga parasite.
The Chainsaw Majini was first encountered by Chris Redfield and Sheva Alomar during their mission while making their way through the streets of Kijuju, who knocked open a steel door and charged them in an alleyway. The two fought the powerful villager and killed him, obtaining the key needed to open the door he emerged from to continue their mission.
During their exploration of the abandoned oil field used by Ricardo Irving, they were attacked by two chainsaw-wielding Majinis while making their way through the refinery area. A third one attacked them and Josh Stone inside the control area along with a mob of regular Majini, who may or may not be spared by the player.
In the events of "Desperate Escape", Stone and Jill Valentine were attacked by a Chainsaw Majini at the Tricell communication facility, who carried a key. He was killed by the duo, who secured the key from his body and used it to continue their journey through the facility.