Shadow is quote on quote the Ultimate Life-form, appearing as a black and red hedgehog he has a troubling past with a human named Maria who ended up tragically passing away. He is untrustworthy to most of Sonic's friends and Sonic himself but Shadow tries to be an anti-hero in his own way. In the Sonic Fandom he's deemed as being edgy. 

I imagine his voice being slightly edgy and low (not too low though), I don't want him sounding too emotionless since there are moments he shows emotions.