Marcus and Didyme were two gifted vampires who met when the Volturi was first formed between the 1400-1200 BC. The two planned to leave the coven to leave behind the violent lifestyle; Didyme was consequently murdered by her brother, Aro, who wanted to keep Marcus for his power.
He traveled as a solitary nomad before he came across Aro, who was newly transformed. He grew fond of Aro's enthusiasm and befriended him. When Aro wanted to form a coven with Caius and Athenodora, Marcus was hesitant. He didn't trust the intense and bitter Caius, but Aro persuaded him.
He had hoped that Didyme would have a gift similar to his mind-reading. A gift she developed, but to her brother's disappointment, it merely worked in inducing happiness in others. Her power only served in bringing the first members of the guard: none of which lasted more than a few centuries. However, he still considered her his infallible ally... until she met and fell in love with his partner, Marcus. She was used to suitors as a human and as a vampire, but none ever caught her attention until Marcus.
Didyme and Marcus first met when Didyme's brother, Aro, first changed Didyme. Her power of happiness induction provided her many pursuers, but no one caught her interest, except for Marcus, who had the power to sense emotional bonds between people. The two eventually fell in love.
When life within the Volturi started to depress them, the two decided to leave the coven to find a more peaceful lifestyle. Aro gave them his blessing, and then murdered his own sister in order to keep Marcus in the coven.
This act, however, became utterly disastrous; Marcus tried to find the 'killer'. When he gave up, he became suicidal. Aro worked to keep him alive, but his efforts were sporadic and dimming with success. Fortunately, Aro discovered Chelsea and her power to influence emotional bonds between people during this period, and quickly utilized her gift to keep Marcus loyal to the coven. This resulted in Marcus losing the desire for suicide. However, he became completely platonic; utterly uninterested in just about anything in the world, even lacking any enthusiasm for the Volturi's affairs. Corin was found sometime later, and Aro offered to use her gift to lift his pain. This, however, did not work out as Marcus did not want to lose his pain.
Marcus remains platonic even after two thousand years and will continue to be so.
Breaking Dawn - Part 2
In Alice Cullen's clairvoyant vision of a battle commencing in a confrontation between the Volturi and the Cullen family, Marcus only observes the fight. When Stefan and Vladimir prepare to make their move on Marcus, he stays in place and holds out his arms, muttering "Finally" (meaning he was finally able to die and join Didyme). The two vampires tackle and split him apart.
This future is eventually changed and Marcus remains alive.