Shadow x Metal is a fanon couple added by Jassiecat600 
Metalsonicshadowboom (1)


Sonic Rivals 2 showed more in depth about the duo's relationship. Shadow at times asks Metal if he's ok even though he can't really talk. When Metal ripped open his chassis, Shadow told him not to because he would destroy himself. It seems these have a deep connection with each other despite the two mostly being on opposite sides.