Kids who grew up watching Sailor Moon undoubtedly remember that the Sailor Scouts undergo a highly choreographed routine to transform that requires the use of cute transformation trinkets--and the Planet of the Fools manga has adopted this to some degree but in a much different way. Instead of cute trinkets, the users in Planet of the Fools must wield weapons called regalia that both grow in size and fuse with their bodies upon transformation, and rather than donning adorable sailor uniforms, these users transform into hideous-looking demons.
In Sailor Moon, Earth's solar system is protected from evil and injustice by magical warriors known as Sailor Scouts who each represent a planet from that system. These warriors live double lives and blend into society by wearing civilian clothes, but upon wielding their various transformation trinkets, they morph into their powerful Sailor form.
All of the Sailor Scouts except for the titular hero Usagi originally used transformation pens to morph before these pens were destroyed and replaced with the more powerful Star Power Sticks. Since Usagi just had to be special and stand out from her Scouts more than she already did, she originally became Sailor Moon with her Sailor Locket before it broke. It later "evolved" into the Crystal Star Locket after merging with the Silver Crystal, an object so powerful that it once allowed the people of the Moon to live long, peaceful lives. Using this new locket was not as easy in the manga as it was in the anime, since it never merged with the Silver Crystal and required Serena to place the crystal inside it in order for her to transform.
Unlike the Sailor Scouts who each represent a planet within Earth's solar system, a certain subspecies of Earthlings in Planet of the Fools called Slarsians can transform into hideous demon-like monsters by unlocking pacts with their personal regalia. Each regalia is different. For the main character Shinto, his is a family heirloom, a knife called Meltz that grows into a sword and fuses with his arm when he triggers the first pact. Meanwhile, the second pact for all Slarsians involves transforming into a hideous monster.
Slarsians can also unlock other abilities such as summoning some type of energy shield and allowing their regalia to evolve. For example, one Slarsian's massive blade splits open to fire lasers. Additionally, Slarsians made specialized armor from nanomachines called Offard that can adapt to the shape of each pact, providing enhanced strength and durability. Offard Armor can also grow wings and relay information through the visor.
Although the direct correlation between both manga is the use of trinkets to help certain characters transform, Planet of the Fools is much darker than Sailor Moon, adding a layer of dynamics to the Slarsians that forces them to face a heightened amount of adversity and bigotry. While the Sailor Scouts have always been beloved (except by their enemies) in every rendition of the franchise, the Slarsians are hated and ridiculed for their "rotting skin" by the Earthians, who, like the Slarsians, are a subspecies of Earthlings. Meanwhile, the Slarsians are exploited by an alliance known as the Federation, forcing many of their kind to later radicalize by becoming separatists and launching a revolutionary campaign. That's like if the Sailor Scouts rose up against Queen Serenity, the ruler of the Moon Kingdom. It's hard to see something like that ever happening.