Feel the Magic: XY/XX

Unlock Chao Hairstyle
Insert the GBA cartridge, Sonic Advance 3, into the DS.
(Thanks to nidoking73@hotmail.com)

Unlock Hard Mode, Body #2, Body #3 and the Sound Test
Beat Normal mode.
(Thanks to l_is_real@hotmail.com)

Unlock Hell Mode
Beat Story mode on Hard difficulty.
(Thanks to l_is_real@hotmail.com)

Unlock NiGHTS Wig
Insert the GBA cartridge, Chu Chu Rocket or Sonic Pinball Party, into the DS.
(Thanks to nidoking73@hotmail.com)

Unlock Sonic Wig
Insert the GBA cartridge, Sonic Advance or Sonic Advance 2, into the DS.
(Thanks to nidoking73@hotmail.com)

Unlock Ulala Hairstyle
Insert the GBA cartridge, Sonic Battle or Space Channel 5: Ulala's Cosmic Attack, into the DS.
(Thanks to nidoking73@hotmail.com)