Metal Sonic: Metal Sonic was created by Dr Robotnik and looks just like Sonic.  Robotnik later sends the mech out to kidnap Amy Rose.  After that Sonic has to go save the little hedgehog in addition to his already large task of defeating Robotnik and Metal as well as destroying the robo machines that Robotnik has planted in Little Planet's past.  Metal Sonic was only in a few games and is a poor imitation of Sonic The Hedgehog.