Ren

Ren is a character in Kingdom Hearts: Heartspawn. He is a ghostly warrior who resembles one half of Blain's heart.

Post KH:H
Little about Ren's human life is unknown, except that he was a great swordsman. At some time in his life, he lost his heart. His Heartless was one of the Armored Knights slain by Sora in the Battle of One Thousand Heartless. Although his heart was released from the heartless, the heart was unable to find its way to Kingdom Hearts. The heart, wandered about the world until it settled in Blain's body, turning the baby into a Heartspawn, some one with two hearts melded into one.

KH:H
In Kingdom Hearts: Heartspawn, Ren revealed himself to Blain in a vision, after Blain was knocked out by Xagna. He told Blain his story, then told the boy to take his hand, thus giving Blain his powers. Blain awoke with the spell, Sabre and the attack, Sonic Gun. He was able to defeat Xagna using these powers.

During his second battle with Axon, Blain met Ren again in another vision. When Ren questioned Blain if he was ready to give up, Blain refused. Satisfied that his young host had such a strong will, Ren taght Blain Sonic Ultima, which alowed him to defeat Axon.

Powers and Abilities
Although Ren never actually fought in KH:H, the fact that Blain's Heartspawn powers (ex. sonic powers) suggests that Ren once used these abilities as some sort of magic. He carries a sword and told Blain of his warrior statis, suggesting that he was once a war hero.