Birthday: secret
Favorite food: sweet things, booze
Least favorite food: none
Favorite color: black
Favorite flower: cherry blossom
Best recipe: anything
Favorite hobby: astonishing people
Likes: the unforeseen
Wishes: secret
What do we know about Clow Reed? Almost nothing. If there's a mysterious, unexploited character in the series, that's him. His whole person is a walking enigma. Clow is the son of an Englishman and a Chinese woman. Besides, her mother belongs to a legendary, powerful sorcerer clan from China. This clan still exists, and the Li family, Syaoran included, descends from it, hence they're Clow relatives.
Reed was a powerful magician, one of the best from his generation (if he wasn't the best). He used high-level magic. Something very charateristic from him is the utilization from both Eastern and Western magic schools. That's because of the cultural mix in his roots, evidently. The Cards have their name writen in both Chinese and English. Sakura "releases" her key in glorious Engrish.
We knew he lived for a while in Japan, in Tomoeda to be precise, until his death. The mansion where Eriol resides was Clow's house (didn't he had déjà-vu's there? o_o).In the manga we don't get to see the wizard's personality, but there's an anime episode full of Clow's past goodness and flashbacks. We could realize Clow was a pacific, reflexive guy, smiled all the time, was fond of Reeding and nature, was quite intelligent and wise, and he loved his creations: the cards, Kero and Yue.
We also know about his clairvoyance. He could see the future way before it'd happen. He knew when he was going to die. He knew who would be the new mistress, and that's why he left the respective arrangements done. Nothing was random. That's why the book was in the Kinomotos' basement. That's why the capture staff is pink. That's why the Moon Bell was left in the Tsukimine shrine. Etcetera. Pretty much everything it happened was according to Reed's plans. "There are no coincidences". In the manga, Clow was overwhelmed with that power of his. It was so unbearable, he wished not to be the most powerful magician. [highlight for manga spoilers] And that's why in manga, he reincarnates in 2 persons: Eriol Hiiragizawa and Fujitaka Kinomoto.