Ibuki is the main protagonist of the story and a descendant of the royal family that once ruled the island nation of Nil Kamui. He was born a twin, alongside his sister Inori, as a scion of this imperial lineage. However, due to prophecies from an oracle, he was hated by society and destined for execution from a young age. To save him, his mother, Hana Guya, arranged for Ibuki to go into hiding and be raised by his grandparents. He lived a quiet life until an encounter with the berserk guardian deity known as the Red Dragon awakened him to the tragedies of the world.

At the beginning of his journey, Ibuki is portrayed as pure of heart, a pacifist who believes violence solves nothing and wishes only for a peaceful existence. Despite this gentle and somewhat naive disposition, he is far from ignorant. He is skeptical and observant, often playing his cards close to the chest while assessing the true intentions of those around him, especially when dealing with untrustworthy allies. He is placed in an incredibly difficult position where he must navigate a world of political intrigue and betrayal while struggling with a terrible power.

His primary motivation stems from a desire to change the world for the better and stop the chaos caused by the Red Dragon. Knowing his true heritage, he joins the Revolutionary Army, hoping to eventually reclaim his throne and win back Nil Kamui's independence.

In the story, Ibuki serves as the designated heir to the throne, acknowledged by the Red Dragon as the Child of the Contract. He becomes the representative of the Revolutionary Army in the Red Dragon Expeditionary Group, a joint mission involving forces from the warring nations of D'Natia and Kouran. His role is defined by his relationship with the Red Dragon; he is forced to wield its power, which comes at a devastating price.

His most significant relationship is with his twin sister, Inori. While Ibuki is the acknowledged heir, Inori is the true Child of the Contract, and he is merely meant to be her help. The Red Dragon's madness has heavily influenced Inori, causing her to become unstable and view killing as trivial, a stark contrast to Ibuki's own values. He also shares a deep bond with Eiha, who serves as his personal guardian. Their relationship is described as that of platonic life-partners, with Eiha constantly by his side to protect him. Another key figure in his development is Mashiro, a young girl for whom he cares deeply, and whose fate becomes a tragic turning point for him.

Ibuki undergoes significant development, evolving from a reluctant and fearful boy into someone capable of accepting the harsh consequences of his choices. Initially, he views the Red Dragon's power as a curse he desperately wants to be rid of. However, as the series progresses and he is forced to sacrifice those he cares about, he learns to shoulder the burden and slowly builds the personality and resolve of a true king. He is forced to make impossible decisions, such as killing Mashiro after she transforms into a Returned One, an act that causes him immense sadness but demonstrates his growing maturity.

Ibuki possesses a unique and tragic ability as a Koutoushu. By forming a contract with the Red Dragon, he can gain incredible power, specifically the ability to manipulate fire, for a short time. However, this power operates on a strict principle of equivalent exchange: he must sacrifice the life of a person he cares about to unleash the dragon's power and take the life of an enemy. This ability is a direct curse, forcing him to trade a friend's life for victory, which is the source of his immense psychological torment.