Didyme

Didyme was a vampire and the wife of Marcus. She was a member of the Volturi until she was killed by her brother Aro. Her death greatly upset Marcus and caused him to become an apathetic being. Didyme was gifted with the ability of making people happy.

Biography
Didyme was born several years after her brother Aro. A decade and a half after Aro became a vampire, he personally transformed Didyme into one; hoping that due to their relation, she would possess an extremely useful talent like his. However, her gift was simply that those around her became incredibly happy. Though this was not what he had hoped for, Aro still planned to put such a power to use.

Because of the aura of happiness around her, many people fell in love with Didyme. However, she only ever reciprocated such feelings for one man; Marcus, Aro's most trusted partner in his growing coven, the Volturi. The pair became incredibly happy together, and to such an end they no longer cared much for Aro's plans to take control of the vampire world. After centuries with the Volturi, the pair made to leave. Though Aro pretended to give his blessing, he murdered Didyme when he knew no one would ever find out so that Marcus would stay (his talent being useful for Aro's cause). Aro truly loved Didyme, but was willing to sacrifice her, and any of his loved ones, for power.

After Didyme's death, Aro had Chelsea use her talents to force Marcus to feel loyalty to the Volturi, though he became a zombie-like being (similar to Bella in New Moon during the period without Edward) who had no passion for anything. Marcus had never learned that Aro was behind Didyme's death. It is quite possible that Marcus entered his dull state due to a withdrawal from Didyme's aura of happiness.

Behind the scenes
On her website's Breaking Dawn FAQ, Stephenie Meyer described Didyme's life as such:

''"Once upon a time, a fairly young vampire (he had only been a vampire for a decade and a half) named Aro changed his young sister Didyme, who had just reached adulthood, into a vampire in order to add her to his growing coven. Aro always wanted power, and because he himself had a potent mind-reading gift, he hoped his biological sister would also be gifted in a way that would help him rise in the vampire world. It turned out that Didyme did have a gift; she carried with her an aura of happiness that affected everyone who came near her. Though it wasn't exactly what he had hoped for, Aro pondered the best ways he could use this gift. Meanwhile, Aro's most trusted partner, Marcus, fell in love with Didyme. This was not unusual; given the way she made people feel, lots of people fell in love with Didyme. The difference was that this time, Didyme fell in love herself. The two of them were tremendously happy. So happy, in fact that, after a while, they no longer cared that much about Aro's plans for domination. After a few centuries, Didyme and Marcus discussed going their own way. Of course, Aro was well aware of their intentions. He was not happy about it, but he pretended to give his blessing. Then he waited for an opportunity to act, and when he knew he would never be found out, he murdered his sister. After all, Marcus's gift was much more useful to him than hers had been. This is not to say that Aro did not truly love his sister; it's just that a key part of his personality is the ability to destroy even what he loves in order to further his ambitions. Marcus never found out that Aro was responsible for Didyme's death. He became an empty man. Aro used Chelsea's gift to keep Marcus loyal to the Volturi, though not even Chelsea's gift could make Marcus show any enthusiasm for it."''