Warp Monarch and Other Aliens
The Cosmic Alliance Warp Monarch are an interstellar monarchy with various planets under them. The reason for Warp Monarch destroying Earth's ancient civilization millennia ago was because of Bosquito running rampant on it. However, Belzeus, claiming that the humans are descended from the Bosquito, using the Gransazers to encourage the Warp Monarch to finish the global genocide they started long ago. Here are other aliens that either worked independently or for the other alien alliances.

Gig Fighters
The foot soldiers of the Warp Monarch, the Gig Fighters spawned from special devices often carried by its high-ranking members. They are often used to hassle the protagonists, or other duties such as supervise prisoners.

From Planet Shiyori, he has the power to enter peoples' bodies. He attempted to use the body of professor Hijikata to have Horiguchi leave the Gransazers. But the failed and Sturcus decides to possess the professor and destroy the team from the inside-out, only retreat after being hit by the Gransazers' Formation Zero attack. Episode 25.

The underhanded Bisilians are sent to destroy Earth, sending an operative and a spy to test the first Stage in their dimensional transport system, using it to bring humans where they eventually disintegrate instantly. After the Gransazers learn of this plot, the Bisilians kill their operative before retreating to begin Stage Two.

Asami Yoshioka
A Bislian spy who assumed the guise of Asami Yoshioka, a girl who died a six months ago of illness. She was said to hinder the Gransazers. From being with Ken and befriending him, Ayumi realizes that Earth shouldn't be destroyed though she is force. After Ran, Ayumi apologizes for deceiving him and is mortally wounded by a Bizal Soldier for betraying her race, revealing the location of the Dimensional Transport system as she thanks Ken with her dying breath, reduced to wisps of light.

Bisilian Mothership
Once learning the location of the Dimensional Transport Machine, the others go to fight the Bisil Soldiers while Tarious activates the machine's self-destruct. Soon after, Garuda engages the Bisilian mothership in an aerial dogfight and destroys it.

A robotic alien from planet Kilardo, he assumes the form of a man in black and is sent to destroy the crystal recorder in the possession of Professor Aya Stacey, the eighteen year old Noble-Price winner. Once he succeeds, he tries to kill Aya and Gou when Tarious and Dail intervene before Tragos delivers the deathblow on the alien robot with Tarious finishing the Kilardoian off. Episode 31

Ramon is from the Planet Stone. He tries to destroy the earth. Ramon killed Kanuma Kouzirou and it is discovered by the Gransazer, he takes his true form and is defeated.

The ghost of an alien soldier who died during Warp Monarch's war against Earth. Filled with undying rage towards humans and unaware that he died, Fedora possessed the body of Henry Wakasugi to continue the orders he received ages ago. It was once his identity is revealed, Tarious uses the crystal slab to make Fedora realized that he's dead, leaving Henry's body to finally rest. Episode 32

A alien from planet Gadaru armed with the Dead Spear, he was a scientist sent by Warp Monarch to convert humans into mindless slaves, using Ran's friends as test specimens in his research. But Ran's friends are freed from the Gadaruian's control as Tarious battles the alien and kills him with Visuel's aid. Episode 35.

A alien felon with no relation to Warp Monarch, he escaped to Earth to feed on the energy there. Armed with a pistol he stole from Freedo's partner, Kriminel can absorb any heat-based energy source to heal himself and increase his power, firing stored energy from the orbs on his chest. After losing to Dail, Kriminel attracts the Fire Tribe's attention as Tenma and Mika fight him and worsen the matter. But once Dail breaks Kriminel's barrier, Freedo delivers the death-shot. Episode 36.

A Warp Monarch agent from planet Gorfin, he enters the body of Mick the dolphin at the Sea Paradise aquarium to influence the other dolphins with his hyper frequency to attack humans with sound waves. But thanks to Mayu, Mick exorcises the Gorfinian as Gans fights before Tarious and Gorbion arrive and the dolphins use their own hyper frequency to weaken the alien as Gans finishes him off. Episode 37.

A Warp Monarch android created in the Omega Project, it as anti-Gransazer weapon hidden within Earth after the battle eons ago in case the Gransazer resurface, to activate after seeing all twelve Gransazers to self-destruct. Uncovered from an Croatian tomb and brought to the JSDF Weapons Institute, Omega came to life and when Mika touched him, having no memory of his past. He later began to explore Tokyo in search for Mika so she can help him gather data of the present-day society. By the time he meets Ran, Omega regains his memories of the past and is compelled to fight Mithras, unable to kill her. Confused, Omega decides to report to Warp Monarch, ordered to gather all the Gransazers after his AI is reprogramed without feeling and at full power. After getting the JSDF's secrets, he battles the Flame Tribe, Dail, Tragos, and Visuel before assuming a giant monster form to destroy Yuuhi. Mithras fights Omega in Garuda until Daisazer is formed and heavily damages Omega, restoring his AI as he apologizes to Mika and runs off. Appears in Episodes 38-39.

The Bosquito is an alien that absorbs any lifeform in its grasp for nourishment, also to reproduce asexually and able to assimilate the energy of its clone spawn when they die. The Warp Monarch couldn't control the Bosqutio as it came to Earth, resulting in the birth of the Gransazers to counter it and the War that destroyed ancient human civilization. Though nearly wiped out, the Bosquito was sealed away within the Kabuto Mountain until Luka revived it on Belzeus' order with no idea who she was reviving until it was too late. Being able to use any attack used by the Gransazers, the Bosquito overpowers the Flame Tribe. After evading the Wind Tribe, the Bosquito reproduces a child before fighting the Gransazers and is killed by them. The surviving child began to feed when Makoto and Jin arrive and were attacked. Tawlon joined in before the Bosquito multiplies itself.
The Flame Tribe arrive to counter the second Bosquito until Yuuhi blasts it. Absorbing its clone, the Bosquito takes the controls for Yuuhi and uses it against the Gransazers, stealing Garuda when the Flame Tribe summons it. Levithan and Gunceasar are summoned to weaken Garuda so eject the Bosquito, who starts to absorb the floral life energy to regain its strength and create two clones of itself. After the others destroy the clones, the orbs they left behind are reabsorbed into the original Bisquito as it evolves into a giant monster. After Tarious uses one of Lion's Crescents to take out the ruby on its chest, the source of its power, the Bosquito is destroyed by DaiSazer. However, as revealed in the Seishin Crossover (Chousei Kantai Sazer-X: The Movie), another Bosquito existed within the monster Bulgaro, released when the monster was destroyed at sea, absorbing the JustiPower in Sazer Island so it can disguise its duplicates as impostor Justirisers. They are all destroyed by Gransazers, Justirisers and Sazer-X but quickly absorbed into the phantom ship so that it would form into Mammoth-Bosquito dragon. The Mammoth-Bosquito is destroyed by Great-Lio, which the power of Gransazers, Justirisers and Sazer-X combined into the finishing attack to destroy the monster. Appears 42-45.