Idomu Yudaiji is a character who appears exclusively in the anime adaptation of CLAMP School Detectives, serving as a primary antagonist before undergoing a significant change of heart. He is the eldest son of the Yudaiji family, a group whose wealth is second only to that of the Imonoyama family. In his early childhood, his parents separated, and his father gained custody while forbidding his mother from seeing him. During one summer, he was permitted to visit his mother at a resort, and it was there that he first met and befriended Nokoru Imonoyama, though the two boys did not learn each other's names at the time. They often played together under a hiba arborvitae tree, until one day when Nokoru fell and scraped his knee. Idomu's mother bandaged the wound and smiled at Nokoru, a rare smile that Idomu cherished and had wished to receive for himself, leading him to resent Nokoru for taking something he considered precious.

Idomu is a mature and highly intelligent young boy with burgundy hair and olive-green eyes, typically seen in the Clamp School uniform. His personality is shaped largely by his deep love for his mother and the loneliness he felt after being separated from her, a feeling he would deny when questioned, asserting that as a boy he was not lonely. This loneliness and longing for his mother's affection transformed into a deep-seated grudge against Nokoru that lasted for years. Despite this, he is capable of showing kindness, such as when he returns a stray volleyball to a group of girls or retrieves a little girl's balloon from a tree, even offering his own form of comfort when it eventually floated away. He is also described as very kind, good at sports, and talented at the piano, an instrument he truly loves.

Motivated by the belief that Nokoru stole his mother's smile, Idomu transfers into the sixth-grade class of Clamp School's elementary division with a clear goal. He vows to take away something that Nokoru holds most dear, specifically the Clamp School itself, and begins causing a series of incidents designed to torment Nokoru and challenge the Clamp School Detectives. In the story, he acts as the primary rival and a formidable adversary, using his considerable intellect and skills to create elaborate problems for the detectives. His role culminates in a major incident where he takes Utako and Nagisa hostage in the campus's subway system, forcing a direct confrontation with Nokoru.

Idomu's key relationships define his character arc. His relationship with Nokoru Imonoyama is the central focus of his story, evolving from childhood friendship to bitter rivalry and finally to reconciliation. Though he initially enjoyed Nokoru's company, the incident with his mother's smile turned his affection into a grudge that drove him to torment his former friend years later. After Nokoru saves both of them from a runaway train, Idomu is able to let go of his hatred, and the two boys reconcile. His relationship with his mother is equally important; he loved her immensely, and her smile was of paramount importance to him. His resentment toward Nokoru was born from the pain of seeing that smile given to someone else when he felt deprived of it himself.

Throughout the anime, Idomu shows some development, moving from a bitter antagonist to a reconciled friend. After the confrontation on the subway, he announces that he will leave Japan to study piano abroad, indicating a fresh start and a focus on the music he loves. Regarding his notable abilities, Idomu is a prodigy with a wide range of talents. He is highly skilled with computers, fencing, and archery, and is also proficient in various other sports according to other students. He knows how to ride a motorcycle and has a liking for making paper airplanes, but his greatest artistic talent is playing the piano, which he pursues with genuine passion.