|Name: Sokichi Narumi|
|Designation: Kamen Rider Skull|
|Weapons: Skull Magnum|
|Gear: Lost Driver|
|Sokichi Narumi is the father of Akiko Narumi and the founder of the Narumi Detective Agency in the fictional city of Futo, eventually taking on Shotaro Hidari as his protege. While working as a private investigator, he became involved in investigating cases involving Dopants that the police force could not handle, including the armored car robberies performed by the "Beastman". Throughout his career, his mantra was to keep the client safe at all costs. At some point, Narumi becomes involved with the Sonozaki Family and is granted a special Gaia Memory and Memory Driver belt, enabling him to become Kamen Rider Skull. He eventually began to regret his work with the Sonozakis and the Museum, with Shroud supporting him by creating the hangar under the Narumi Agency before having him infiltrate a Museum research facility known as the Gaia Tower with Shotaro in order to find the young man at its center. After fighting several Masquerade Dopants and the Taboo Dopant, he and Shotaro find the young man as Narumi offers him freedom to find atonement for his part in the creation of the Gaia Memories, naming him Philip after his favorite fictional character Philip Marlowe. After Narumi frees Philip from the Gaia Tower and begins to take both him and Shotaro out of the building, he is shot by several armed guards.
As he dies, he gives his white fedora to Shotaro and asks for Shotaro to become a man worthy of wearing it. It was after the Gaia Tower fell into ruins that Shotaro and Philip, spurred by his advice to continue in his place, learn that Sokichi's body is no where to be found. In Begins Night, Sokichi Narumi and Kamen Rider Skull are among the identities taken by the Dummy Dopant while pretending to be the Death Dopant in a scheme to assume the identities of Futo's upper class members, using Sokichi's form to drive Shotaro off from the case before he realizes the truth. However, Shotaro finally overcomes this and Double ultimately defeats the imposter Kamen Rider Skull. After the case is concluded in Movie War 2010, Tsukasa Kadoya uses a Kamen Rider Skull card he has acquired to summon an iteration of Sokichi Narumi from an alternate dimension, giving Shotaro closure. This Sokichi Narumi does not seem to know Shotaro but recognizes the very thing that the real Sokichi wanted of Shotaro: for Shotaro to become a true detective.
In Kamen Rider W Forever: A to Z/The Gaia Memories of Fate, he gives Shotaro his Lost Driver before fading away as he takes his leave, enabling Shotaro to transform into Kamen Rider Joker. The Skull Memory is one of the 26 T2 Gaia Memories in the film. Koji Kikkawa as Sokichi Narumi/Kamen Rider Skull appeared in the film Kamen Rider × Kamen Rider OOO & W Featuring Skull: Movie War Core in late 2010, with the full story behind Narumi becoming Skull being revealed in the first segment.
By using his Gaia Memory in the Lost Driver belt, Sokichi Narumi is able to transform into Kamen Rider Skull. As Kamen Rider Skull, he still wears his signature white fedora and also wears a white scarf. After the hat is damaged in his fight with the Taboo Dopant, it appears torn, even as a disguise used by the Dummy Dopant. As Kamen Rider Skull, Narumi's strength is increased and he also has access to a unique motorcycle and is armed with the Skull Magnum gun. Before entering a fight, he announces "Now, count your sins!", a motto taken up by his protege. Like any of the Kamen Riders in Double, his weapon is capable of performing a Maximum Drive finishing attack capable of destroying Gaia Memories. Kamen Rider Skull Crystal is the incomplete form of Skull that first appears in the events of Kamen Rider × Kamen Rider OOO & W Featuring Skull: Movie War Core. It differs from Kamen Rider Skull's standard form in that the helmet is iridescent and it lacks the S-shaped "crack". This form is named after the crystal skulls.