Reia Hiruda is a grade 10 student and a member of the student council at St. Luminous College, distinguished by her long dark hair, which she often wears in braided twintails, her fair skin, dark eyes, and a stature of 160cm and 45kg.

She appears as an innocent and quiet girl, deeply devoted to the student council president, Haruto Saionji. Together, they discovered a book called 'End Earth' that transported them to an alternate world. In this world, Saionji becomes the emperor of the Arms Alchemy Empire, Baelgard, under the name Traihard. Reia’s desire to follow him is complicated by her refusal to be consumed by the darkness of his mission, leading her to develop a dark second personality known as Hildareia, the Hellfire Alchemist. As one of Baelgard’s four guardians, this alter ego embraces the ruthless actions Reia wished to avoid.

In her Hildareia persona, she becomes treacherous, nearly insane, and conducts cruel experiments on elves to transform them into mindless soldiers for Baelgard’s army. Wielding the Mythical Beast Sword Mysteltain, a violet chain sword, she serves as a powerful offensive force. This dark personality eventually betrays Saionji by aligning with Killrain after he takes control of the dark elven army. The only method to restore Reia’s original self is to destroy the evil personality’s soul blade.

Once restored, Reia rescues Saionji when his life is endangered while linked to a dark tower. This act reveals her true feelings to him, and he reciprocates, developing his own emotions for her. Their connection enables Saionji to draw his Ultimate Soul Blade, the Holy Grail, from her heart, which becomes essential in defeating a powerful enemy.

In the aftermath, Reia chooses to remain in End Earth with Saionji, Xecty, and Jin-Crow, staying in Baelgard to help restore the forest to its former glory.