Maria is a demon who has a beast inside her that she can barely control. She asks Blade, a powerful warrior, to kill her if she ever loses herself. Blade agrees, but he also wants to help her unleash her full potential.
They go to a school where they meet Yessica, Sophie, Cu, Earnest, and Leonard, who are all friends and allies. Maria tells them her secret and they promise to support her. Blade challenges Maria to release all of her power at once, and she does.
She breaks the five barriers that limit her power and creates a huge storm. Her power is over 160 times higher than normal. She can fly and easily defeat Earnest and Leonard. Sophie tries to attack her with The Demon King, a powerful weapon, but Maria deflects it. Cu breathes fire at her, but she compresses it and throws it back.
Blade fights her with his sword, but he keeps the scabbard on. He wants to capture her alive. Maria senses this and smashes him into the ground. Blade decides to get serious and unleashes a flurry of attacks, but Maria blocks them all and shoots an energy beam at him. Blade blocks it with his sword, but it breaks.
Earnest gives Blade another sword, called Asmodeus. Blade uses it to strike Maria with Holy Demon Sword, a powerful technique. Maria tries to stop it, but she can’t. She is defeated.
Blade faints and wakes up in the medical unit. The doctor tells him that he was lucky to survive. The king tells him that Maria is the daughter of the Demon King, who left her and her mother to fight an enemy.
Earnest gives Maria a nickname, Mao. Yessica says that he misses Maria. Mao comes to Blade’s house for a date. She says that she likes him and jokes that she won’t leave until she kills him. She also says that Blade only used 15% of his power.
Mao reveals that Maria doesn’t want to take off the necklace that keeps her power in check. She also reveals that she is a different personality that Maria created to cope with the trauma of being abandoned by her father.
Eliza tells them that the necklace is also a recording device and might bring back Maria. Mao agrees to try it. They play the recording and hear the Demon King apologizing to Maria’s mother for leaving her behind. He says that he had to fight a foe. Maria learns the truth and forgives him. Mao says goodbye and faints. Maria comes back.
In the end, Mao and Maria coexist and share their body. Mao lets Maria take over when they fight against Blade.
Maria is a new character who has a hidden demon side. She created Mao to deal with the pain of losing her father, who left to fight a powerful enemy. This adds more mystery and suspense to the story, as the viewers wonder what kind of enemy he faced and how Blade will deal with Mao and Maria.
Maria finds out the real reason why her father left and accepts it, which is an emotional moment in the story.