Zodiarts

The Zodiarts are the seconday antagonists in Kamen Rider: Quadronaut. Each of the Zodiarts is themed after one of the 88 Western constellations, & each Zodiarts' body is decorated with the pattern of the constellation it is named after. The name "Zodiarts" is derived from the English word "zodiac".

Background
Reverse engineered from the Core Switch to reach the alien race known as the Presenters, the Zodiart Switches are designed to force-evolve the user, the Switcher, into a being that can survive the void of space. The result of the transformation is a Zodiart, a monstrous construct of Cosmic Energy.

After the Zodiart Switch is used several times in a row or when the Switcher is enraged enough, it will announce "Last One" & the Zodiarts Switch will change its appearance, growing spikes & an eyeball-like structure, as well as changing the position of the button. When the Switcher activates the Switch at this point, the transformation becomes irreversible as the Switch drains its Switcher of their conscious mind & uses it to become a Zodiart permanently. The Switcher's empty body is ejected from the Zodiart, covered in a cobweb-like substance. In this state, a Zodiart becomes extremely powerful to the point that its destruction would cause a massive explosion that would destroy everything surrounding it. A Zodiart, especially one at its strongest, can only be defeated if targeted at point of its body where the Zodiart Switch is located. From there, Kamen Riders Quadronaut or Meteor can use one of their Limit Break finishing attacks to destroy the monster & get the Zodiart Switch.

Once properly switched off, the Zodiart Switch will vanish into a singularity, keeping its user from ever becoming a Zodiart again. However, it is still possible for a Switcher to become a Zodiart again through another Zodiart Switch. It also appears that a Switcher gains an addiction to Cosmic Energy after using a Switch, which only helps in making them Zodiart again. Also, if a former Switcher has obtained Last One previously, their new Zodiart Switch will automatically assume it's Last One form. Combined with the threat of evolution, this would be a quick way for some Horoscopes to be born through former Switchers.

Zodiarts are collectively named after the Western Zodiac.

Horoscopes

 * Main article: Horoscopes

The end result of Lucius Hope's research for a means to reach the Presenters, the Horoscopes are evolved Zodiarts that transcend human nature. A Horoscope manifests whenever a Resurrection Star on the body of a Zodiart in its "Last One" stage glows. Once the evolution happens, once brought to it, the Zodiart's original human body is turned into Cosmic Energy & absorbed back into the Zodiart's body as the new-born Horoscope molts out of its old Zodiart body as it dissolves into Cosmic Energy. In some cases, the evolution occurs faster with those who already obtained the potential & need to only use a Zodiart Switch once to transform, bypassing the Last One stage. Hope's goals are related to gathering all twelve Horoscope Switches together.

Orion
Kevin Lahti is a member of the MWHS football club who became tired of how the team's members, particularly captain Humphrey Grant, treat him as their slave & never let him have a chance on the field. To get back at Humphrey, he is given a Zodiart Switch that allows him to become the Orion Zodiart, a Zodiart with intense brute strength. Although he is initially defeated by the first appearance of Kamen Rider Quadronaut by the Kamen Rider's Rocket Drill Kick, the mysterious Scorpion Zodiart returns the Zodiart Switch to Lahti, allowing him to use it once more to confront Humphrey. After Amity Muhlfeld reveals the identity of the Zodiarts to Guy & Letitia Dunbar, they reach Lahti in time to convince him not to go through with his plans. However, Humphrey pushes Lahti over the edge by kicking him out of the football club, driving him to use the Zodiart Switch just as it enters its Last One state, also arming his Zodiarts form with the Lemnos Club & the Chios Shield, though he loses both weapons during the ensuring fight with Quadronaut. As the monster has amassed enough energy to blow up the stadium, Jeong Thrussell directs Kamen Rider Quadronaut to take the Orion Zodiart into the upper atmosphere in order to destroy him with the Rocket Drill Space Kick, allowing the Zodiart Switch to be turned off (after re-entering the atmosphere).
 * Switcher: Kevin Lahti; male Foundation X member
 * Defeated by: Kamen Rider Quadronaut Base States' Rocket Drill Kick, Rocket Drill Space Kick, Kamen Rider Double Detective CycloneJoker's Joker Extreme
 * Constellation Base: Orion

A second Orion Zodiart appeared later on, this time an unnamed adult male affiliated with Foundation X, sent to defeat the gathered Kamen Riders. He lated defeated by Kamen Rider Double Detective.

Chameleon
Perlie Rayne is a third-year student who is in the cheerleading squad, in addition to being one of Kanani Weaver's "sidekicks". However, Perlie has become tired of Kanani's two-year winning streak in the annual Queen Festival, a contest that votes for the "queen" of the student body. To stop Kanani from winning her third year so she would be able to win the contest herself, Perlie is given the Zodiart Switch that allows her to become the Chameleon Zodiart, a Zodiart that can appear invisible as well as attack with its powerful tongue-like tendrils.
 * Switcher: Perlie Rayne; male Foundation X member
 * Defeated by: Kamen Rider Quadronaut Base States' Rider Rocket Drill Kick, Kamen Rider Double Detective LunaTrigger's Trigger Full Burst, Kamen Rider Meteor's Meteor Strike
 * Constellation Base: Chamaeleon

To orchestrate her plan to remove Kanani from the contest, Perlie begins to attack students that could be potential threats to her success. Then, when Kanani takes the stage for the talent competition, Perlie fakes being attacked herself so she would be able to transform & cause a commotion, allowing for a video of Kanani lambasting the student body to play. When her identity is revealed, Perlie fakes remorse to catch Kanani off guard & injures her severely to remove her from the contest. However, Kanani musters up enough strength to return to school despite her yet-to-recover injuries & wins back the hearts of the student body with her powerful speech.

Perlie, enraged that her plan has failed, ignores Jeong's warning against using her Zodiart Switch when it enters its Last One state. She attacks Kanani as the Chameleon Zodiart once more, but Kamen Rider Quadronaut is able to save the school's reigning "queen" & eventually destroys the Zodiart with his Rider Rocket Drill Kick, gaining access to the Zodiart Switch to shut it off. After Kanani wins the contest for the third year in a row, she forgives Perlie for her actions, & renews their friendship under the promise that Perlie would beat Kanani through honest means.

A second Chameleon Zodiart appeared later on, this time an unnamed adult male affiliated with Foundation X, sent to defeat the gathered Kamen Riders. He is later defeated by Kamen Rider Double Detective.

A third Chameleon Zodiart appeared & was believed to be Perlie who got a new Zodiart Switch. However, what remained after a fight with Quadronaut in Elek States & Meteor was a clump of hair, & it was later revealed to be a fake created by the Coma Zodiart.

Unicorn

 * Switcher: Fingal Neville; female Foundation X member
 * Defeated by: Kamen Rider Quadronaut Elek States' Rider Ten Billion Volt Break, Kamen Rider Double Detective CycloneJoker Xtreme's Bicker Charge Break
 * Constellation Base: Monoceros