Fran, a girl of the Black Cat Tribe, met Teacher—an Intelligence Weapon sword—and became his wielder. Belonging to the Adventurer's Guild in the fortress city of Alessa, Fran has completed numerous quests and raised her reputation as an adventurer. Fran aims for the 'Evolution' that lies beyond strength. Teacher, wishing to fulfill her desire, set their next destination as the port city of Seedrun. While heading to their destination, Teacher and Fran discovered a 'Floating Island' in the sky and met Jean Du Vix, a necromancer aiming for a complete conquest of the island. They accepted his request to challenge the Floating Island's dungeon together, but... It was a malevolent labyrinth swarming with undead. Despite appearances, she has a dim-witted but kind personality.

Originally a Japanese human on Earth, he was an ordinary thirty-year-old office worker who loved curry, anime, and manga. After dying in an accident, he was reincarnated in another world as an Intelligence Weapon (essentially a giant Zweihänder equipped with a general artificial intelligence close to an artificial consciousness) and a growth-type Magic Sword-class armament. He looks after Fran, who is inexperienced in many ways outside of combat, and Jet, who is usually like a large puppy, acting as their guardian. Both Fran and Jet trust and follow Teacher.

Ten years ago, after hearing news of frequent undead appearances in the plains far from Alessa, he investigated the cause and discovered it was due to necromantic floating debris falling from the Floating Island. Realizing the island was an undead dungeon, he attempted a solo conquest but failed and escaped. Since then, he has been preparing various means to conquer the dungeon. He is the person the Fran party met after being attacked by a Dead Knight while flying to the island and falling. Since the party landed in the cultivation room of Jean's laboratory, they unintentionally ruined some of his Necro-grass. As compensation for the damaged grass, he proposed the conquest of the Floating Island dungeon, or if that proved too difficult, the capture of a Magic Beast Wraith Absorption, which the Fran party accepted. He promised to give Magic Stones for Teacher as a reward. His profession is a Spirit Master, an advanced class of Necromancer, and he fights by summoning and commanding undead-type magic beasts. Furthermore, he doesn't just summon fixed undead; he is deeply knowledgeable in the creation, cultivation, and modification of undead, creating various types to respond to different situations. Because he doesn't use undead as mere disposables but considers their future—namely, a satisfying passing to the afterlife—the undead he commands are often sincerely loyal to him. His unique skill, Soul Eye, allows him to see the target's soul directly. It cannot be blocked by Appraisal Block. Through this skill, he realizes Teacher is an Intelligence Weapon. However, it cannot be used on undead-type magic beasts. A demon the party met while traveling to the port city of Seedrun, he is a necromancer who calls himself the King of the Undead. Though his arrogant speech and behavior can lead to misunderstandings, he is a quite good person and an active adventurer.