 Seven returning characters from previous Sonic titles star in Sonic Lost World . The protagonist , Sonic the Hedgehog who must defeat the main antagonist Doctor Eggman and stop the Deadly Six . Aiding him in his quest is Tails , Sonic 's fox friend who has the ability to fly . Knuckles the Echidna , Sonic 's strong friend , and Amy Rose , his self @-@ proclaimed girlfriend , also make relatively minor appearances . 
