Eriol Hiiragizawa is best known as the main antagonist of the second half of the series, often called the Master of the Clow arc. He's the reincarnation of Clow Reed, carrying both the original magician's memories and his full magical power. Like his predecessor, Eriol has his own magical staff and can command Yue and Cerberus—Clow's original guardians. He's also created a matching pair of guardians in Ruby Moon and Spinel Sun.
Eriol transferred to Tomoeda Elementary from England after Sakura Kinomoto became the official master of the Clow Cards by passing the Final Judgement. At school, he presents himself as a quiet, mature student, though classmates know him as a troublemaker—he and Takashi Yamazaki are constantly cooking up elaborate lies about the history of random objects and places just for fun. While Sakura doesn't suspect him, Syaoran Li is wary from the start. Things get dicey when Yue and Cerberus recognize Eriol's magical signature as Clow's, forcing Eriol to erase Yue's memory after a chance encounter.
The real reason Eriol came to Japan was to push Sakura into converting the Clow Cards into Sakura Cards using her own power. The cards were slowly dying because they were drawing energy from Clow, but Sakura's magic couldn't reach them until she performed the conversion herself. Yue faced the same problem—Sakura's brother had to constantly feed him magical power to keep him and his human form, Yukito, alive.
Sakura doesn't figure out that Eriol's been orchestrating all the magical disasters until near the end, when he finally reveals himself. He puts the entire city to sleep and challenges her to convert the last two cards—the Dark and the Light—at the same time. After she pulls it off, he tells her she's now stronger than Clow Reed ever was and asks her to do one final thing: split his magical power between himself and her father Fujitaka, who is the other reincarnation of Clow Reed. He brings Sakura, Syaoran, Tomoyo, and both guardians back to his place to explain why all this was necessary—converting the cards without a driving reason could've caused them to spiral out of control or drained Sakura's life force entirely.
Eriol's actual age is a mystery. He used magic to stop his aging so he'd stay the same physical age as Sakura. He's in a romantic relationship with Kaho Mizuki, the woman he originally sent from England to help Sakura with the Final Judgement. By the end of the manga, Kaho returns to England with him, and they're shown together in the second anime movie when Sakura calls him for advice.
The anime adaptation changes things slightly. Eriol is the only reincarnation of Clow Reed in that version—all the stuff about Fujitaka also being a reincarnation and receiving half his powers got cut. The scene at Eriol's house where he explains everything happens the day after the final battle instead of right after, and it happens because Sakura seeks him out rather than him inviting everyone over.
Content compiled by AnimeList.moe from publicly available sources.
