Cain was an extremely huge, bulky man with dark hair. During Throne of Glass, his size and power increased, effects of his use of Wyrdmarks and the Valg's power to consume his victims' power. He also wore a black Wyrdstone ring, inducing the Valg to live inside him.
A denizen of the White Fang Mountains, Cain was a skilled warrior. To add on to this, he displayed blatant ambition and bloodlust, willing to harm or kill any who stood in his way.
Several months after his death, Aelin wonders if Cain fought against the Valg who eventually possessed him via the ring.
Throne of Glass
Cain was from the White Fang Mountains and descended from the local freefolk. He was found by Duke Perrington (the mountains and Anielle were part of Adarlan) and used as his candidate in the royal tournament. Cain murdered several of his rivals by calling on the ridderak using Wyrdmarks. As a result, Cain gained the strength of his opponents and magical powers, which helped him during the tournament. In the final round, he almost defeated Celaena Sardothien, but with Elena's help, she triumphed over him. Cain then attempted to murder her, but Chaol saw what he was doing and killed him to defend Celaena.