The cruel villain (whom turns out in the end not to really be a villain, but I dislike him anyways.) Taishakuten, whom started out the God of Lightning, and then the Emperor of Tenkai. Taishakuten had an affair with Shashi, Ashura-Ou's wife in order to gain intelligence needed to destroy the Ashuras. Also (typical to the CLAMP storyline) was in love with Ashura-Ou. Ashura-Ou used this to his advantage, and made him keep a promise that is they key to the whole series. But, I ain't spoiling it; so you gotta wait and see.
Shashi
The cruel and beautiful mother of Ashura and Ten-Oh. Shashi was originally a miko of the Ashura clan, but she was not happy with the position; and married Ashura-Ou to gain more power. This didn't satisfy Shashi's thirst for power, so she had an affair with Taishakuten, who later made her Empress of Tenkai. She has tried to kill her son Ashura on several occasions, and doesn't show any love or pity for him.
Ten-Oh
Prince of Tenkai, and a good natured soul (with parents like Taishakuten and Shashi, you wonder how the hell that happened). He is sympathetic, kind, and sought by several maidens. However, he only has eyes for Kendappa-Ou whom is in turn in love with someone else...
Kara
Shashi's sister, and complete opposite. She is a kind, gentle, loyal woman whom helps Ashura out in the manga. She was completely satisfied with being a Miko of the Ashura clan, unlike her power hungry sister.