He is Yuu in a future timespace, La'cryma, and is one of the protagonists of the anime. However, though his mission was to find the Dragon Torque, Haruka, Karasu had an alternate reason as he removed his pipeline to remain in our timespace as Haruka's protector. In his timespace, he failed to save Haruka from death, and thus swore to protect the Harukas of other timespaces from anyone or anything. His devotion is such that even torture and mortal injuries cannot stop him; he even briefly goes mad when she is about to die, triggering the memories of his Haruka's death. As a result, he brutally kills one of the other Dragon Knights and smashes open a dam, screaming "Go! Hell calls you!" He even fights his oldest friend, Isami/Fukurou, almost to the death rather than allow Haruka to be harmed.

He is also irritated by the Yuu of our timespace, seeing him as weak and indecisive, although it seems that he despises Yuu for his own inability to save his Haruka. They are actually very similar, with a psychic bond to Haruka, a desperate need to keep her safe, and a tendency towards angst and doubt. Karasu apparently still whittles.

After invading Shangri-la with Yuu, Karasu destroys several of Shangri-la's ships, but is horrified to find that his nemesis Noein is actually another version of him and Yuu. He assists Yuu in disowning Noein's existence, destroying his power and freeing La'cryma.

It is not entirely clear what happens to Karasu after Noein's destruction; by disconnecting himself from the pipeline early in the series, he condemned himself to fade out of that dimension, which would kill him. In the final scenes, he transforms into Yuu after speaking to Haruka, suggesting that he may have merged with Yuu.

Like all Dragon Knights, Karasu's body has been radically altered on a quantum level – when he is injured, the inside of his body shows silver, and he bleeds blue raizu particles. His spin weapon consists of several black whip-like threads that he can control. With them, he is able to create shields, grab distant objects, and cut through most anything. He has also been seen using blasts of built up energy. The spirals on the arms of dragon knights can also become swords, Karasu and Fukurou are seen using this weapon while fighting in episode 12 for a short while. Karasu is the Japanese word for crow.

Due to the fifteen years that have passed, and the strain of his life in La'cryma, Karasu is not immediately identifiable as Yuu. While Yuu looks like an ordinary young boy, Karasu has pallid skin, silver hair that falls over his face, and dresses in a huge black cloak. Despite these differences, his eyes are the same as Yuu's, and Haruka sees the resemblance between them.

(Source: Wikipedia)