An armed bank robber with a violent past was sought by a gang of mutant criminals called the Mutnoms. One of its operatives asked him if he can be a monster if he wants to get away with every crime, including stealing and killing. With his body permanetly altered, little does that bank robber, named Takeru Wasabi, know that he is losing his humanity to the wrong people, and was picked up by mysterious authorities. A mysterious police task force called H.U.M.A.N. sentenced Takeru by giving him a device that is too dangerous for humans. And Takeru agrees, hoping to make the Mutnoms pay for tricking him. When Mutnoms were wrecking havoc and carrying out their criminal activities, Takeru Wasabi becomes Kamen Rider Sirven! And is about to bring those monsters to justice!
Takeru Wasabi/Kamen Rider Sirven: the season's protagonist who was once a messed-up criminal wasting his life robbing and killing people to let him survive. When the Mutnoms trick him into being a monster, he was afterwards arrested by H.U.M.A.N., and was chosen to be Kamen Rider Sirven. He is very sarcastic and perverted.
Zuko Ryuga/Kamen Rider Gunner: the only secondary rider of the season, and his Kamen Rider motif is a vigilante, modeled after crime-killing gunners like the Punisher and Paul Kersey. Zuko is very vengeful, anti-social, and has a very violent past about an evil crime gang killing his beloved family. With nothing to live for, he becomes the self-made Kamen Rider Gunner, dedicating his life killing criminals and terrorists who would harm innocent people as a way to get what they want. He calls every member of H.U.M.A.N. "wusses", believing that they are not doing the job right.
Patroller: Sirven's motorcycle used for pull-overs and high-speed chases and pursuits.
Sirdriver: Sirven's henshin-belt that has 7 slots(like how Decade has 1, W has 2, OOO has 3, Fourze has 4, Wizard has 5, and Gaimu has 6)for special devices called the VenButtons.
Magnumren: Sirven's side weapon capable of great bullet fire and blank fire to apprehend suspects. It also becomes the Magnaton.
ShockPunchers: In Shock Form, this is Sirven's boxing gloves that produce heavy-metal punches, also used as charges. Its special final attack commences when the Shock Venbutton is inserted.
BlastExton: In Burn Form, this is Sirven's gun/extinguiser/flamethrower hybrid weapon that can fire fireballs and put out fires. Its special final attack commences when the Burn Venbutton is inserted.
SpeedSaw: In Turbo Mode, this is Sirven's chainsaw-like weapon that can cut through anything, even solid rocka nd steel. In super Speed, he can use that weapon to his advantage. Its special final attack commences when the Turbo Venbutton is inserted.
BrightKendo: In Stealth Mode, this is Sirven's beam sword with a bright glow, and can shock victims and can cut through steel. With it, he can strike foes with it without notice when invisible. Its special final attack commences when the Stealth Venbutton is inserted.
Devilasher: Gunner's own motorcycle modeled after a Harley-Davidson motorcycle.
UneDriver: Gunner's henshin belt with infrared technology that activates features when the KillCrusher is scanned with a certain Venbutton inserted, with the Skull Venbutton as an exception.
KillCrusher: Gunner's own firearm weapon capable of laser fire and rapid fire. Depending on what Venbutton is inserted, special bullets/traps will fire, including the final attack.
Venbuttons: strange devices that plug in the Sirdriver for Sirven to change into many forms. They have buttons that perform special features. There are also other Venbuttons that have different features like use of weapons and gear.
- Siren Venbutton: Venbutton that turns a small siren on when the button is pushed. It only goes on the center of the Sirdriver, and Takeru transforms into Kamen Rider Sirven when he pushes the button of this Venbutton of the switch after inserting it in.
- Shock Venbutton: Venbutton that gives Sirven his first form: Shock Form. It consists of a yellow rubber suit so he won't get electricuted. He uses this form to crack cases and trip up crooks when in situations involvig lightning storms, power plant accidents, etc.
- Burn Venbutton: Venbutton that gives Sirven's second form: Burn Form. It consists of a red fireproof suit he uses during battles and investigations in situations that involve fire and fire triggers. It can also protect him from toxic gas.
- Turbo Venbutton: Venbutton that gives Sirven's third form: Turbo Form. It consits of a purple suit with jets that increases his super speed so much that even surpasses Patroller's speed when it wasn't enough. It was used for high-speed pursuits.
- Skull Venbutton: Gunner's own Venbutton that activates his transformation by pushing its button near the UneDriver.
- Stealth Venbutton: Venbutton that gives Sirven's fourth form: Stealth Form. It consists of a green suit that gives him special goggles for night vision and tracking invisible targets. With it, he can also become invisible and sneak in places without being detected.
- Aqua Venbutton:
- Zeyphr Venbutton:
- SWAT Venbutton:
- Masked Venbutton:
- Gun Venbutton: Venbutton that summons the Magnumren. This Venbutton can plug inside the weapon in order to make it useable.
- Battery Venbutton: the most powerful common Venbutton of all that allows final attacks to be launched, but it has to be recharged after each use. Gunner has that Venbutton too.
- Smoke Venbutton: Venbutton that causes a smokescreen.
- Stun Venbutton: Venbutton that stuns a target.
- Grenade Venbutton: Venbutton that fires an open grenade.
- Illusion: Venbutton that fires a gas that causes targets to hallucinate.
- Hook: Venbutton that fires a grappling hook.
- Magnet: Venbutton that allows Sirven to stick to magnetic/metal surfaces.
- Booster: Venbutton that gives Sirven super strength for a few minutes.
- Shield: Venbutton that envelopes a forcefield around Sirven for a few seconds.
- Rope: Venbutton that ties crooks around.
- Light: Venbutton that acts like a flashlight.
- Spray: Venbutton that blinds and burns eyes of crooks. Also can be used to find hard-to-find evidence, like fingerprints and footprints.
- Signal: Venbutton that produces an ear-splitting jamming signal to disrupt devices and machines, and to deafen enemies.
- Spark: Venbutton that fires fiery bullets from the KillCrusher.
- Ice: Venbutton that allows KillCrusher to entomb enemies in ice.
- Flash: Venbutton that makes the KillCrusher blind crooks so easily.
- Net: Venbutton that helps the KillCrusher fire a paralyzing net.
- Cherry: Venbutton that allows the KillCrusher to fire cherry-bomb-like bullets taht explode on contact to deal extra damage.
- Acid: Venbutton that allows KillCrusher to fire acid on any surface, even steel, rock , and concrete.
Mutnoms are an organization of criminals who want to destroy and wreck havoc to expand their criminal empire, even with means of mutant science. Mutnom operatives keep searching around Japan to find a decent host. They recruit, manipulate, and turn most of Japan's criminals into brainwashed Mutnom monsters themselves. And they go around commiting crimes, such as robbery, murder, rape, blackmail, hacking, and even minor terrorism. Takeru was too turned into a Mutnom, but he was lucky to be not brainnwashed.
Mysterious Figure/Mr. E: mastermind of all Mutnoms shrouded in a big trenchcoat, fedora, and sunglasses. His identity and agendas are a mystery, and is yet to be revealed soon. Later, he is revealed to be the Mutnom crime boss Mr. E. Mr. E is creating Mutnoms to create havoc to strengthen is criminal empire.
Maseru Kiba/Baboon Mutnom: the Mysterious Figure's eyes and ears. He is finally defeated by Sirven.
Getrude/Flower Mutnom: Mr. E's assassin and Osama's sister. She is named after Getrude Banizewski. She is very cunning.
Osama/Lion Mutnom: Mr. E's assassin and Getrude's brother. He is named after Osama bin Laden. He is very sarcastic.
Hitmen: masked grunts of the Mutnoms, armed with guns.
- Mantis Mutnom (1-2)
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