Katsumi Daido

Katsumi Daido is the leader of the NEVER terrorist group and the main antagonist of Kamen Rider W Forever: A to Z/The Gaia Memories of Fate as well as the main protagonist of Kamen Rider W Returns: Kamen Rider Eternal.

Kamen Rider W Returns: Kamen Rider Eternal
Katsumi Daido's mother was Miki Daido, a researcher for Foundation X studying Necro-Overs to duplicate the conditions that revived Raito Sonozaki / Philip. After Katsumi fell victim to a fatal car crash, Miki used her research to save her son by converting him into a Necro-Over.

However, the Foundation rejected Miki's research in favor of the Museum's research into Necro-Overs, causing Katsumi and his mother to leave the Foundation. The two later revived four other people who had recently died into Necro-Overs and together they founded the mercenary group NEVER.

During a mission to South Asia where NEVER wa fighting a terrorist organization, Katsumi encountered a Quark named Mina and fell in love with her. Katsumi later met with Jun Kazu and obtained the experimental Eternal Memory, allowing him to become Kamen Rider Eternal.

However, after Doctor Prospect killed all of the Quarks, including seemingly Mina, Katsumi went insane before killing Prospect with Memory's Maximum Drive. His Eternal Memory damaged, Katsumi discarded it and decided to wipe out the Museum.

Kamen Rider W Forever: A to Z/The Gaia Memories of Fate
Katsumi attacked the helicopter Tabata was riding to transport the briefcase of T2 Gaia Memories to Foundation X's base. Katsumi took the T2 Eternal Memory and transforming into Eternal before stealing the entire case and goes to his accomplices. However, Tabata activates a switch to cause the briefcase and helicopter to explode, causing the Memories to rain down across Futo City. Katsumi escapes the explosion and goes with his men search for the remaining Memories.

Katsumi later appears at the place the Riders came to meet Maria S. Cranberry and attacks Kamen Rider Double, overpowering him in both his FangJoker and CycloneJokerXtreme forms. After defeating Double, defeated, Katsumi takes the 19 T2 Gaia Memories the Riders found. After capturing Phillip and obtaining all 26 of the T2 Gaia Memories, Daido initiates the X-Bicker to send out a wave to turn everyone into Necro-Overs, despite knowing that they will die due to not being as enhanced as he is. However, Philip and Shotaro reactivate their Gaia Memories and mount a counterattack against Daido. As this happens, Maria betrays Katsumi and injects with a serum to cause him to start dissolving. However, Katsumi shoots Maria and takes another serum to stabilize himself before escaping and leaving her for dead.

The Riders soon catch up to Katsumi, but he quickly escapes again. Double then catches up again to Eternal on the Futo Tower and fights Eternal. As Double CycloneJokerXtreme is about to finish off, Eternal goes into his Strengthening Armament mode and uses the Zone Memory to summon the other T2 Gaia Memories. He then uses the Never Ending Hell Maximum Drive to knock Double off the Futo Tower and send him plummeting to his death, but through the support of the citizens of Futo and a strong gust of wind blowing into the Xtreme Memory, Double changes from CycloneJokerXtreme to CycloneJokerGoldXtreme. Eternal casts a blast of energy at Double, but Double counters the attack and delivers a kick to Eternal powerful enough to defeat him and 26 T2 Gaia Memories at once. With his very last breath, Katsumi expresses that he remembers what death feels like before dying.

Posthumous
Katsumi's backstory was later told to Shotaro and Philip by Mina. After hearing it, the two lamented how Katsumi could have become a heroic Kamen Rider and protected Futo City had his encounter with Prospect not driven him insane.