Erieol Sin is the name used in some adaptations for the character originally known as Eriol Hiiragizawa, a pivotal figure who appears in the second major story arc of the anime. He is introduced shortly after Sakura Kinomoto becomes the new master of the Clow Cards, transferring to her elementary school in Tomoeda as a new student from England. His precise age is ambiguous; while he appears to be a boy of about eleven or twelve, he has used his magic to halt his aging process, allowing him to join Sakura's class as a contemporary when the time was right. In truth, he is the reincarnation of the Clow Reed, the legendary sorcerer who originally created the Clow Cards. As such, he possesses all of Clow Reed's memories, vast magical power, and a deep understanding of the cards and their guardians, Yue and Cerberus.

On the surface, Erieol presents himself as a kind, polite, and exceptionally gifted student. He is academically brilliant, athletic, and skilled in everything from cooking to playing the piano, and he wears his characteristic glasses with a gentle, often smiling demeanor. He quickly befriends Sakura and her friends, often joining classmate Takashi Yamazaki in weaving elaborate and fictional histories for everyday objects. However, this affable exterior masks a more complex and cunning personality. He can be mischievous and seems to genuinely enjoy the challenges he creates, though he must sometimes restrain himself from making knowing comments when Sakura and Syaoran Li are slow to understand his intentions. Despite his strategic and occasionally manipulative actions, his motives are not malicious. His fundamental nature is kind and deeply caring, especially towards Sakura, whom he watches with a very gentle and protective gaze. He is known to dislike causing her genuine sadness.

Erieol's central motivation and role in the story is to act as a hidden benefactor who forces Sakura to grow into her power as the true mistress of the cards. After Sakura passed the Final Judgment, the Clow Cards began to lose their magical energy because they were still tied to their original creator. To prevent them from fading away, Erieol creates a series of magical disasters that compel Sakura to convert each Clow Card into a new card powered by her own star magic. He deliberately plays the part of an antagonist, setting tests and obstacles that appear dangerous but are carefully designed to help her master her abilities. His ultimate goal is to relieve himself of the burden of being the most powerful sorcerer in the world, a life he found joyless. In the manga, his wish is fulfilled when Sakura uses her magic to divide his immense power, giving the other half to her father, Fujitaka, who is revealed to be Clow Reed's other reincarnation.

His key relationships are central to the plot. He holds a profound and ultimately romantic connection with Kaho Mizuki, who had previously served as a teacher at Tomoeda Elementary. He created his own pair of magical guardians, counterparts to Yue and Cerberus: the serious and cat-like Spinel Sun, and the playful, powerful Ruby Moon, who often takes the human form of a high school girl named Nakuru Akizuki. His relationship with Sakura is that of a mentor and challenger, while Syaoran Li is immediately suspicious of him, sensing the powerful and ancient magic he exudes, and also grows jealous of his easy friendship with Sakura. Erieol is perceptive of Syaoran's romantic feelings long before Syaoran himself fully understands them, and he subtly creates situations that help advance their relationship.

Throughout the story, Erieol undergoes significant development. He begins as an enigmatic and seemingly antagonistic figure whose true purpose is a mystery. His motivations are gradually revealed, culminating in a final confrontation where he confesses his identity and his plan to help Sakura. After his goal is achieved, he drops his facade completely, showing his true, warm personality. He returns to England with Kaho Mizuki and his guardians. He later reappears in the Clear Card arc as a distant but invaluable ally, using his powers from abroad to investigate a new magical threat, Yuna D. Kaito, and to provide crucial support and information to Sakura and her guardians when they face a new set of mysterious cards.

Erieol possesses a wide array of notable magical abilities. As a reincarnation of Clow Reed, his power is immense, considered among the greatest of living magicians. He wields a magical staff and is capable of casting powerful spells, such as shrouding the entire city of Tomoeda in an impenetrable darkness and putting all non-magical beings to sleep. He can animate inanimate objects like toys and amusement park rides, perform levitation, and unleash powerful energy blasts. He also has the ability to create magical guardians, see spirits, and possesses a form of premonition, although this ability is weakened after he gives away half his power in the manga storyline. His power is so great that magic based on the moon, over which Yue presides, has no effect on him.