The villains of Advent Children are the "Sephiroth Remnants" (Kadaj, Loz and Yazoo); the remains of Sephiroth's will and spirit in physical, "larval" form. They seek out Jenova's head, unknowingly spurred on by Sephiroth's will with an obsession to be with their "mother", through which Sephiroth can be reborn. They each reflect an aspect of Sephiroth's character, and refer to Cloud as their Big Brother. They are able to summon monstrous embodiments of "tainted lifestream" called Shadow Creepers to their aid and apparently are weak against the effects of Lifestream-influenced water.

Yurie Niku Sakamoto is a fan character created by our very own Yurie and Aswari Carter. (Ownership of the character is to Yurie though).
She is the twin sister of the Turk Aswari and despises her sister. She is determined to destroy her sister, as shown by her actions time and again, and is deeply in love with Sephiroth. (For further details, see her history profile).