Didyme

Didyme was Marcus's wife and former member of the Volturi until she died at the hands of her brother, Aro. Her death greatly impacted Marcus, who became apathetic afterwards. Didyme had the gift to make people happy, also known as the aura of happiness.

Biography
Didyme was born several years after her brother Aro. A decade and a half after Aro became a vampire, he transformed her into one as well, hoping she would show a useful talent similar to his. However, her gift was simply to make those around her incredibly happy. This notwithstanding, Aro still planned to put such a power to use.

Because of the aura of happiness, many people fell in love with her. However, she only ever reciprocated those of Marcus, Aro's most trusted partner. The pair became incredibly happy together, and to such an end they no longer cared much about Aro's plans to gain control of the vampire world. After centuries with the Volturi, the pair made plans to leave. Aro pretended to give his blessing, and murdered Didyme so that Marcus would stay (his talent being more useful for Aro's cause).

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 while without Edward), with no passion for anything. He never learned that Aro was behind Didyme's death. It is quite possible that Marcus' dull state is an effect of his withdrawal from Didyme's aura of happiness.

Powers and abilities: happiness induction
Didyme had the power to release an aura of happiness, which could bring joy to herself and everyone in her surrounding, freeing people from sadness and making them adore her. This power is similar to Jasper Hale's gift to manipulate the emotions of others, but it only worked in one direction.

Marcus
Didyme first met Marcus when her older brother Aro brought her into the coven. Though Didyme had many admirers, she never paid them any mind until she was a mature vampire and met Marcus. They deeply fell in love with each other. They were so happy being together that Aro's ambition for domination of the vampire world became increasingly less important to them. Soon enough they began talking about leaving the Volturi to lead a life together. Needless to say, Aro was well aware of these plans. During a battle with another coven Aro took the opportunity and secretly killed Didyme in order to keep Marcus in the Volturi. Marcus was totally debilitated by her death and became suicidal but Aro had Chelsea utilize her talent to tie Marcus to the Volturi. Marcus never recovered from Didyme's death. In Breaking Dawn: Part 2, when Marcus was killed by the Romanian Coven , he said, "Finally.", with a small smile on his face. It can be assumed he was glad to die, after being unable to end his life for such a long time, and wanted to see Didyme again.

Aro
Didyme was the younger sister of Aro and they had a very close bond. Aro changed her into a vampire about one decade after himself. Because Aro developed a strong gift as a vampire, he hoped that by changing Didyme, she would develop another to his advantage. To his disappointment, her gift wasn't as useful as he had wanted.

She was very loyal to him until she and Marcus fell in love. They were planning on leaving together, but Aro valued Marcus's gift too much and murdered Didyme to stop their plan.

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, 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."''