Marcus is one of the three leaders of the Volturi, and is considered vampire royalty. He has jet black hair and the typical red eyes of a vampire, he has the slightly odd translucent skin of the three leaders.
After Marcus became a vampire, Aro's biological sister, Didyme, fell in love with him. Didyme had a supernatural talent that made people happy, literally. They married, and planned to leave the Volturi to start a life together. Aro, who pretended to give his blessing, snuck in and murdered Didyme, to keep Marcus from leaving the Volturi as he was too important to Aros plans. Marcus was never told what Aro did, and to stop him from running away, Aro used Chelsea to stregthen the bonds between them.
 New Moon
In New Moon, after Edwards suicide attempt after beliving that Bella was dead, Edward and Aros telepathic conversation revealed that Marcus was surprised at the intensity of Edwards and Bellas relationship, which reminded him of his own relationship with Didyme. Bella described Marcus as looking eternally bored, never showing any signs of emotion. This could be because of Didymes death, Marcus had nothing to live for.
 Breaking Dawn
So why was Marcus kept in the Volturi? Surely he has no use there? Well, according to Stephenie Meyer, Marcus has the talent to see the binds between people, but cannot manipulate them, unlike Chelsea. "Marcus's gift is actually quite valuable in the hands of someone who knows how to use it. For example, imagine Marcus on the battle field. He sees the binding ties between the enemy the guard faces. He can tell who the leader is just by watching the way the others are tied to him or her. He can see how to destroy the resistance simply by picking off a few key people. He doesn't just see romantic relationships, but also ties of family and friends, loyalty and devotion. He can tell if any given person would die for another. Off the field of battle (as the Volturi rarely leave their home), Marcus's gift is valuable politically (for lack of a better word). It's an amazing gift in a diplomatic situation. Plus, when you're sitting on an empire, it’s nice to be able to see when someone's loyalty starts wavering. Or if anyone is getting too fond of anyone else. Can you see the potential now? Marcus didn't get where he is with some fluff skill." - Stephenie Meyer, on Marcus.
 Film Portrayal
Christopher Heyerdahl plays Marcus in the movies.