Reaper Legion are a team of mid-evolved Roidmudes who eliminate any Roidmudes who refused to follow Heart and supporting antagonists in Kamen Rider Drive. They were created to replace Mashin Chaser as the "Grim Reaper" of the Roidmudes after Medic altered his base program to defend Roidmudes, rendering him unable to kill any.
Reaper Legion's first mission saw Roidmudes 044 and 094 being sent to eliminate Roidmude 072, a Roidmude who wish to live in peace with humans rather than fight against them. 072 was killed off by Medic, causing Kamen Rider Drive Type Dead Heat Flare to attempt to attack her with a Dead Heat Drop Full Throttle. However, Medic shielded herself using her two Reapers, resulting in their destruction.
A team comprised of Reaper Legionnaires Roidmude 034, Roidmude 054, and Roidmude 104 was later formed by Medic to cover Chase's escape from the Special Investigation Team. 104 was terminated by Kamen Rider Mach's Signal Kaksarn-powered Beat Macher soon after Medic, 034, and 054 abandoned him. While Drive was still recovering from the whiplash Type Formula gave him, Kamen Rider Mach fought the remaining two Reaper Legionnaires when Medic picked up Chase when he was starting to revert back into his old Grim Reaper self, destroying them with a Heat Kick Macher.
The Freeze Roidmude also requested the services of a couple of Reaper Units from Medic, who provided him with Roidmude 028 and Roidmude 092. Appearing in human form as aides of Freeze's persona, National Bureau of Defense secretary "Soichi Makage". When Shinnosuke confronted "Makage" after discovering his true identity, the Reapers assumed their true forms and attacked Shinnosuke who, having come without Mr. Belt, was at a disadvantage until Chase showed up to fight them. Transforming into Kamen Rider Chaser, Chase destroyed both Roidmudes with the Across Breaker.
Medic was later accompanied by the Reaper 041 when she interrupted Drive Type Tridoron's attack on the Cook Roidmude, making him the latest test subject for her "Super Evolution Experiment". When Medic using Cook's golden sauce, Honfleur's Dusk on him, he began to go haywire and burst into flames. When he tried to sacrifice himself to let Cook finish both Drive and himself, Heart blocked both attempts, then placed his hand on 041, soothing and stabilizing the effects of the sauce on him.
041 was later summoned by Medic alongside Roidmude 045 to help intercept Kamen Rider Chaser when he was pursuing Cook on his Ride Chaser. They were overwhelming Chaser before Drive Type Formula arrive to even the odds. They were soon joined by Cook, whose fighting capability was enhanced by Medic after consuming the perfected Honfleur's Dusk. With Deadheat Mach arriving as well, determining that the Honfleur's Dusk was important to Medic after she changed the scene of battle, the three Kamen Riders rallied with Deadheat Mach going into controlled Burst Mode while Drive became Type Tridoron. With Drive facing Cook as well as Medic, Mach and Chaser took on the Reapers each, destroying both 045 and 041 using their Full Throttles, starting with Chaser assaulting them with his Across Breaker before Deadheat Mach finished them off using his Heat Kick Macher.
After Tenjuro Banno took command of the remaining Roidmudes upon achieving his Gold Drive form, he sent out a trio of Reapers consisting of 013, 022, and 080 to accompany Medic (whom he had brainwashed) in hunting the Brain Roidmude so he could complete the Promised Number. He later brutalized 013 when he arrived with news of Brain's location late. Alongside the Super Evolved Medic, they attacked Brain, who had disguised himself as a van driver, Brain reassumed his own Super Evolved State to resist them before help arrived in the form of Kamen Riders Drive (Type Technic), Deadheat Mach, and Chaser. Facing the Reapers as Medic and Brain fought evenly matched, the Riders soon defeated and finished them off with the Frostreamer, Zenrin Shooter amplified by Burst Mode, and Shift Max Flare in the Break Gunner. With her back-up gone, Medic retreated, but not before telling Brain that she would be back to take him to the Promised Land.
Medic later sent another trio of Reapers consisting of 021, 032, and 083 to corner Brain while heading for the Promised Land, which was supposedly a lakeside. The Reapers kept the Kamen Riders at bay while the Sigma Circular was activated with enough energy for a second Global Freeze, but with Medic being the sacrifice to avoid the Sigma Circular from overloading, the Reaper trio refused to help due to being made to serve only Gold Drive. Medic was saved, however, by Brain transferring the chip to his own body, sacrificing himself in the process. The Reaper Trio continued their assault on the Kamen Riders until they were destroyed by Chaser's Across Breaker.
The rest of the Reaper Legion gathered at the place Gold Drive called "The Promised Land", the SSDC main building. Roidmude 015, Roidmude 035 and Roidmude 075 were sent by Gold Drive to destroy the Drive Pit to ensure a second "true" worldwide Global Freeze without resistance. However, the Kamen Riders appeared and destroyed the Reapers with their triple SpeeDrop. The second three, Roidmude 015, Roidmude 059 and Roidmude 070, demonstrated their Heavy acceleration on three helpless humans, turning them into data for the Sigma Circular's awakening, as it was Gold Drive's plan to ensure a true second worldwide Global Freeze where any who resist are denied a physical form. This enrages the Riders, now with help from Heart and Medic, as they aim to not only kill Gold Drive but also destroy Sigma before its awakening is complete. As the Kamen Riders fight the second Reaper Legion trio, Heart, Medic, and Drive head upstairs to reach Sigma, while Mach and Chaser fight Gold Drive and the three Reapers. The Reapers are eventually destroyed by Chaser's Across Breaker.
Roidmude 081, Roidmude 101 and Roidmude 105, the final three Reapers, were sent by Gold Drive to once again keep the Riders at bay for the second Global Freeze, but their bodies were destroyed by the combined efforts of Drive, Heart, and Medic. Medic attempts to recover their Cores, but they are helpless and are destroyed instead, hinting it to be some sort of a trap set up by Gold Drive.