Shard was originally just like any other Metal Sonic. He was beaten by Sonic in the end, but unlike a lot of the others that came before him, Eggman gave him upgrades. With each new upgrade, he gained more and more of a personality. More, and more free will. Eventually, he was given a power crystal, which morphed him into Metal Sonic V2.5. He had his own cocky attitude, and was dead set on murdering Sonic. However, his consciousness gave him the truth, as he realized what he was doing was wrong, and sacrificed himself to save Sonic and Tails from a Volcano. 

Some time had passed, and his power crystal was excavated from the rock that used to be the Volcano's magma. It was completely intact unlike his melted body, and still held all of his memories and programming. This crystal was put into a new body made from the scraps of certain other Metal Sonics that followed his creation, and thus Shard was born. Using his free will to help Sonic, and save the world, he now lives as a sentient freedom fighter. He has even grown an attraction to an AI, by the name of Nicole who also helped Sonic.

Shard is more stubborn, and cocky than Sonic, but has his own kind heart. His Voice, should be that of Sonic's but lower pitched, and carrying a slightly different attitude to things. Though he has no Voice, I'd go for a mix of Dante from Devil May Cry and Sonic. You're free to take liberties and give it your own spin. I would also recommend takes with and without a robotic filter for added effect.

https://www.youtube.com/watch?v=PveFajDo2rI&t=18s