Shizue Izawa

A former Japanese who became a demon in another world. She appears as a fragile, beautiful girl with black hair, around 16 or 17 years old, bearing scars from burns, but her actual age is around 70. She was the first otherworldly person Rimuru encountered after his reincarnation, and she became the model for Rimuru's human form. She was once known as the "Ruler of Explosive Flames," a famous hero. In Volume 1 of the book, she heads to Leon's castle to fulfill her only regret, and on the way, she and Ellen are taken in by Rimuru's village where she meets Rimuru. However, the Ifrit residing within her goes berserk, causing her to lose consciousness. Rimuru stops the rampage by using "Predator" to consume the Ifrit, but Shizue's body rapidly ages. In her final moments, Shizue wishes to be consumed by Rimuru, passing on her hatred for Leon and her skill "Transformer" to Rimuru, where she rests peacefully within him. Later, she appears in Rimuru's mind, and in Volume 4, she conveys her lingering regrets about Chloe and others to Rimuru, and in Volume 11, she reunites with Hinata and Chloe.

Volume 1 also depicts Shizue's past. About 60 years ago, at the age of 8, Shizue was summoned by Leon with severe burns all over her body and was possessed by Ifrit. Initially unable to control Ifrit completely, she tragically ends up killing her friends, but after visiting Leon's castle, a female hero gives her an "Anti-Magic Mask," allowing her to control Ifrit's power. She is protected by the hero and together they become adventurers, retiring as an adventurer in her early old age and becoming an instructor in the Kingdom of Ingracia. Yuuki and Hinata are her students, with her last students being Chloe and Kenya. Realizing her life was nearing its end, she embarks on her final journey to confront Leon with her complex feelings of hatred.

Ifrit, the Fire Giant

Voice - Yashiro Taku

A high-ranking spirit that possessed Shizue. After being absorbed by Rimuru, he reincarnates as Veldora's servant in Volume 11. In "Veldora's Slime Observation Journal," after awakening to self-awareness, he reflects on his past failure to reconcile with Shizue and deeply regrets killing Shizue's friend Pirino. According to Veldora, the compatibility between Ifrit, who worships Leon, and Shizue, who hates him, was the worst. If their hearts had been in harmony, their power could have been multiplied, and their lives might not have ended there, suggesting a different path could have been possible. For more about the resurrection, see "#Charis."