Rinne Kannagi is a 17-year-old girl born into a prestigious family that performs the subjugation of monsters known as "Apparitions." The Kannagi family has been entrusted by the government with the vital "Duty" of protecting the citizens' safety, and Rinne has been raised with a strict education since childhood to fulfill her role as the next head of the family. While she is revered like a living god at home and admired as a top-tier student in both academics and sports at school, Rinne has suffered from a mysterious phenomenon since the age of three: she cannot see the faces of others. This has prevented her from making close friends. One day, ordered by the government for her first extermination mission, Rinne uses her powers as a practitioner to summon a god—Yakumo. Unlike others, Yakumo's face is clearly visible to her. Though the reason remains unknown, Rinne heads to the site with Yakumo. As they eliminate the appearing Apparitions, a strange entity with a "Core" appears, regenerating no matter how many times it is defeated. They are saved from this struggle by a mysterious young man named Jugemu, who suddenly appears and shatters the Core—which Rinne and Yakumo could not destroy—in a single blow. His face, too, is clearly visible to Rinne, just like Yakumo's.

In the near future, in a country that has become a dependency of a foreign power, lives a girl named Masamune. "Masamune" is the name given to the head of a clan that has served for generations as guards to the "Mikado," the royalty who have ruled the land since ancient times. She is one who bears the mission of living as the Mikado's protective blade itself. As the 16th generation, this girl possesses talent rare even among her predecessors; despite being from a branch family, she inherited the power passed down through the ages and the "Divine Blade" that emits a red flash, succeeding to the name Masamune. She constantly strives to remain calm and collected to fulfill her duties, but due to her unique upbringing, she is somewhat detached from the world and shows a bit of an airheaded side. While she has vast knowledge of all weapons, including swords, and is familiar with everything from their structure to their usage, she is incredibly clumsy in fields outside of combat. She is particularly poor at handling household appliances. Furthermore, she is a surprisingly big eater, contrary to what her appearance and behavior might suggest, and her favorite food is steamed pork buns.

Lucifer saved Strike World from a crisis at the cost of her own life. Thought to have perished by both the Heavens and the Underworld, she had actually drifted to a different world with her memories lost. A nun named Magdalena discovered and cared for her, bringing her back to a church in the slums for protection. Magdalena lived in the slums under the oppression of a tyrannical Empire. Welcomed by the people there, Lucifer found a moment of peace. However, one day, the Empire's oppression reached the slums, and soldiers arrived to arrest the residents en masse. The tyrant, noticing the immense power sleeping within Lucifer, captured her along with the townspeople and ordered her execution. Just as Lucifer was about to give in to despair before the overwhelming power of the oppressor, her past memories resurfaced. With the spirit of rebellion reignited, the gates of "Cocytus"—the prison of rebels connected to the depths of her soul—were unleashed upon the world. Gaining new power, Lucifer was reborn as the Monarch of the Fallen. After annihilating the executioners in an instant with her overwhelming strength, she sat regally upon her throne. The oppressed people of the slums praised her, their voices rising in joy at the birth of the Savior of Rebellion.

She is described as having a personality that is "curious and clumsy." An innocent and airheaded girl, she makes many mistakes, but her inherent super-positive thinking allows her to overcome any situation brightly, making her a charming character.

She has a flat way of speaking and acting, possessing a sensibility that is far removed from common sense. However, she is essentially very strong-willed and shows a kind side to those she trusts.

She has a bright, energetic, and devoted personality. On the other hand, she is a bit of a hasty and clumsy girl. Her story involves her struggling to do her best as a maid in the human world.

She is serious and has a strong sense of justice, but is also stubborn and reckless. Her personality is the exact opposite of her partner, Ivory. While she often clashes with the insincere Ivory due to her own eagerness to achieve results, they deepen their bond as the ultimate partners.

She is insincere and lazy, but quick-witted and highly cautious. Her personality is the polar opposite of her serious and justice-minded partner, Ebony, but through many clashes and investigations, they have become the best of partners. She absolutely loves seeing Ebony's cute reactions.

An archangel who is quiet, cool, and taciturn, but occasionally shows a playful or mischievous side out of nowhere.

Originally the black reaper of Slavic mythology, in Monster Strike, she is a character with a dangerous charm who mutters things like, "I'm dying of boredom... I wish a cursed, bloody war would break out in the demon realm..." She is a sadistic, demonic "jirai-kei" girl who only wants to do fun things.