At first glance appears to be a friendly and reasonable policeman; but this is just a facade he puts up to conceal his identity as the Commander Dopant. He really has the psychotic belief that criminals are just "trash" with no salvation and the only way to save them is through death, to the extant of becoming a Dopant. He is willing to frame and even kill fellow policemen who get in the way of his beliefs.
Two years prior to the events of W Returns, Sagami was out with his wife on their anniversary giving her a ring until she was shot by a criminal Sagami had arrested. In order to get rid of the "trash" that courts couldn't handle, Sagami purchased the Commander Gaia Memory. When he chased Aoi Katsuragi and her father after the latter had done pickpocketing scheme, Aoi's father attempted to give himself up to Sagami, but Sagami transformed into the Commander Dopant and killed him.
Sagami reappeared after Ryu Terui was called for an investigation about a syndicate of pickpockets. When Ryu found Aoi, Sagami as the Commander Dopant had his Kamen Soldiers attack and placed a bomb on Aoi which would detonate in two hours if she did not bring him the Gaia Memory Enhancement Adapter she stole from him and frames Ryu for attacking Jinno. When the device was recovered, Sagami had his partner, Yukihiro Ohno, distract him while he took Aoi and the adapter.
Ryu used a bus to stop the car they were in and cornered them. Sagami explained his reasons and returned Ryu's Accel Memory Aoi stole from him to kill her. Shotaro Hidari and Philip then interfere, transforming into Kamen Rider Double to fight the Masked Soldiers. Ryu transforms into Kamen Rider Accel to fight the Commander Dopant who initially overpowers him until becoming Accel Trial, disarming the detonator. But the Dopant uses the adapter and becomes stronger defeating Accel.
He then abducts Akiko to convince Ryu on becoming an "executioner" by making him remember the loss of his family. But when Ryu arrives he tells Sagami that he let go of the past and his feelings of vengeance to become a Kamen Rider who protects Futo City. Sagami becomes the Commander Dopant to blow up the platform Akiko is on while Aoi steal the adapter and throws it Ryu becoming Accel Booster and defeats the Commander Dopant with the Booster Slash Maximum Drive.
Sagami is then arrested for his crimes.
As the Commander Dopant, Sagami, with the touch of a button on his left arm, can create Masked Soldiers resembling a SWAT team armed with rods that can give off electricity, and delete people like data. He is also armed with missiles and a timed bomb which can be stopped by deactivating the detonator.
When using the Gaia Memory Enhancement Adapter, the Commander Dopant becomes more powerful with additional armor on his chest with a cogwheel sticking out from the back. In this form, the Masked Soldiers become more powerful and the Dopant can fire more missiles at once.