Sephiroth

Sephiroth is the manifestation of Cloud's darkness.

First meeting
Sephiroth served as the "Platinum Match" for Sora at Olympus Coliseum. Sephiroth was a fierce opponent, possessing powerful attacks, including swings from his massive sword, "Descend Heartless Angel", which sapped Sora's health and magic, and dark orbs. Sephiroth was also able to teleport. After being defeated by Sora, Cloud finally met Sephiroth. Sephiroth implored Cloud to turn to the Darkness, whcih Cloud refused. The two enaged in a high-speed battle to the air, the outcome of which is unknown.

Second meeting
During the Heartless invasion of the Radiant Garden, Sephiroth appeared in front of Sora and asked where Cloud was. Sora was unable to give an answer, so Sephiroth took his leave. After Sora defeated the Master Control Program, Sora found Sephiroth at the Dark Depths. Sephiroth was interested in the identities of the three heroes, and professed interest in the Keyblade and decided to battle Sora for it. Despite Sora hankering for more, Sephiroth stopped the battle and told Sora to get Cloud.

Cloud arrived, and the began battle again, with Sephiroth desiring to draw Cloud into the darkness. However, Tifa protected Cloud, saying that Cloud did have light in him, and that, if he didn't, then he would be surrounded by light. Nevertheless, Sephiroth and cloud st off for another battle which outcome was unknown.

Background
Sephiroth, like Cloud, comes from Final Fantasy VII, in which he was previously a member of the Shin-Ra Corporation's army SOLDIER.