Shadow the Hedgehog is a character that appears in the Sonic the Hedgehog series. He is an anthropomorphic hedgehog/Black Arms hybrid who resembles Sonic the Hedgehog in many ways and serves as his arch-rival and occasional ally. Shadow was created as the "Ultimate Life Form" by Gerald Robotnik by using the DNA of Black Doom, and is the end result of Project Shadow. 

Being Sonic's polar opposite personality-wise, Shadow is, bluntly put, a brooding loner. He usually acts with a cool and businesslike indifference, only occasionally showing his vulnerable side. He is rarely seen with others for an extended period of time and usually distances himself from essentially everyone. He is usually blunt and does not speak much, but he always knows exactly what to say in combat. However, Shadow never bluffs; if he makes a threat, he has every intention of carrying it out. However, he does deeply care for others but has a hard time of expressing it.