Seiichiro Matsui

Seiichiro Matsui, nicknamed Matsu was Sokichi Narumi's partner in 1999 until becoming one of the first Dopants. Matsu used the Spider Gaia Memory to become the Spider Dopant.

Personality
Matsu was close to Sokichi and always helped in his cases by researching certain facts. However, he had a slight envy of his partner always getting the girl. When he fell in love with the opera singer, Melissa after she hired him and Sokichi to save her from Kozo Yaguchi he went as far as to use a Gaia Memory to become a Dopant in order to fulfill that request. After a few uses of the Gaia Memory, Matsu's mind warps to the point of wanting Melissa as his lover by keeping her from touching Sokichi and planting bombs on the people of Futo so that they would lose the ones they loved.

Abilities
Matsu originally spent a lot of time in a library researching facts for investigations, not unlike Philip when using the Gaia Library. After gaining the Spider Gaia Memory to become the Spider Dopant he gained the ability to use threads called the Spider Web to attack and implant the small but destructive Spider Bombs into humans that would spin silk around a person's loved ones and explode on touch. Unlike other Dopants who lived from their Memory Breaks, Matsu died from the Spider Memory's side effects after it was destroyed because the Gaia Memories were still in developmental stages at that time.

History
Sokichi had come across Matsu one day and they became partners for solving cases that the police couldn't handle. One day Melissa came in and asked for their help. Through some means, Matsu gained the Spider Memory to become a Dopant in order to fulfill the job while also stalking her in attempts to make her love him. Matsu would then fight his partner as the Spider Dopant and Sokichi then figures out Matsu's identity after recognizing the way Matsu writes the number "1". After Melissa explains Matu's reasons for taking the Gaia Memory, Matsu becomes the Spider Dopant, kidnaps Melissa and implant his Spider Bombs on the people of Futo City. Chasing his partner to a bridge as Kamen Rider Skull, Sokichi expresses his regret for not seeing Matsu's madness and stopping him sooner and they have one last fight where Skull defeats the Spider Dopant with a Rider Kick Maximum Drive and Matsu dies from the memory's side effects.