Seiichiro "Matsu" Matsui is the main antagonist of the W segment of Kamen Rider × Kamen Rider OOO & W Featuring Skull: Movie War Core. He was the partner of private detective Sokichi Narumi who is secretly the Spider Dopant.
He was portrayed by Taro Yamamoto.
Seiichiro Matsui formed the Narumi Detective Agency with Sokichi Narumi to take cases the police would not handle. At some point however, Matsu received the Spider Gaia Memory from Museum, and began using it to stalk the opera singer Melissa.
Sokichi and Matsu were later hired by Melissa to investigate her stalker, who was unknowingly Matsu, and her seedy talent manager Kozo Yaguchi. In secret, Matsu became the Spider Dopant and attacked the police, fighting Sokichi before fleeing along with the Gaia Memory Distributor Eren Komori. After the incident, Matsu returned in human form and suggested to Sokichi that they leave the investigation up to the police.
However, Sokichi ignored his partner's warnings and, with help from an informant, unearthed the missing girls Kozo Yaguchi had provided to Museum as test subjects for Gaia Memory Research. Sokichi went to confront Yaguchi, but the Spider Dopant killed him remotely with one of his spider bombs.
Despite being seemingly left without any leads, Sokichi managed to deduce that the Spider Dopant's true identity was Matsu from Matsu's handwriting. However, after learning Matsu's true motives, Matsu becomes the Spider Dopant and kidnaps Melissa before fleeing. Sokichi then transforms into Kamen Rider Skull and pursues Matsu to a bridge, where he has a final confrontation with his former partner. Skull defeats the Spider Dopant with a Maximum Drive, causing Matsu to perish from the Gaia Memory's side effects.