Iori Asahina is the tenth son of the Asahina family, a large household of thirteen brothers who come to live with a new stepsister. He is introduced as a young man of eighteen years with an elegant and refined appearance. His slate-gray hair falls across his hazel eyes, and he is typically seen in the crisp uniform of his private school. Standing at approximately 177 centimeters tall, he carries himself with a dignified air that has earned him a reputation as a "prince-like" figure among his peers. He is known for being popular and well-regarded not only at his own institution, the prestigious Bright Centrair Catholic academy, but at nearby schools as well.

On the surface, Iori embodies calmness and tranquility. He is perceptive of others' emotions and often acts in a caring, gentlemanly manner, particularly towards the family's new sister, Ema. He is also the brother assigned to care for the household garden, a task that suits his reserved nature. However, this serene exterior conceals a deeply scarred interior. In his youth, Iori had a girlfriend named Fuyuka Shiraishi, whom he loved profoundly and frequently brought to the Asahina home. Her sudden death in a car accident during his high school entrance exams shattered his world. The trauma of her loss left his heart feeling empty and triggered a severe psychological crisis that included suicidal ideation. He was prevented from taking his own life by his older brother, Kaname, an act that paradoxically turned Iori's grief into a seething hatred for Kaname, whom he began to view as an enemy.

This tragic backstory is the core motivation for Iori's actions throughout the narrative. His psychological state is fragile, and his earlier, gentle personality becomes distorted by his unresolved pain. When he meets Ema, he misinterprets her familial kindness and sympathy as a romantic commitment, leading him to believe she will fill the void left by Fuyuka. This misconception drives him into a state of increasing obsession. He develops a possessive and delusional belief that Ema is dependent on him and that her life is as dark and hopeless as his own. This culminates in a pivotal and deeply disturbing event at his late girlfriend's grave, where Iori attempts to recreate a twisted union, expressing a desire to take Ema with him to "the other side" to be with Fuyuka.

At this critical moment, the truth about a cross necklace he had treasured as Fuyuka's final gift is revealed to be a well-intentioned lie crafted by Kaname, his supposed enemy. This revelation shatters his delusions. In the aftermath, a disillusioned Iori flees to a cliff edge to end his life, but the glint of the very same cross necklace calls him back from the brink, allowing him to begin a path toward recovery. The anime uses the language of flowers to represent his feelings for Ema; he gives her an iris, a flower whose hidden meaning is to "risk everything to love you". It is noted that following his psychological struggles and a period of separation from his family, he later establishes a career as a professional model.

Iori's most significant relationships are defined by his trauma. With his brother Kaname, his arc is tragic; once the brother who understood him best, Kaname becomes the target of Iori's deepest resentment after preventing his suicide and later deceiving him about the cross's origin. In the novels, this conflict escalates to physical violence. His relationship with Ema is similarly fraught, beginning with genuine kindness and evolving into a desperate, possessive fixation that nearly ends in tragedy before it is resolved. Concerning his abilities, beyond his academic prowess and his skill as a gardener, he is noted to hold an initial rank in aikido, underscoring that his composed exterior is backed by a degree of physical capability.