You’ll get to see her nude if you actually beat normal, hard, and hell mode.
Girl on Menu Screen Loses Her Top. Simply set the language to Japanese, and the
girl mysteriously loses her top.
Record your voice on the title screen. While at the title screen, hold Down + Y
to record whatever you want into the microphone. It will now play back whatever
you recorded at random intervals while the title music plays. However, if you
wish to play it back immediately, press Down + X. Down-Left + X will play it
back slowly, while Down-Right + X will speed it up.
Once you beat Normal mode you unlock Hard mode
Beat Normal mode to unlock the sound test.
Beat Normal mode to unlock Body 2 and 3.
Beat Hard mode to unlock Hell mode (along with a Santa and Cowgirl outfit in maniac mode).
Unlockable: Hair (You can unlock different wigs by having certain GBA games inserted in the DS)
Chao hairstyle : Sonic Advance 3 cartridge
NiGHTS Wig : Chu Chu Rocket or Sonic Pinball Party cartridge
Sonic Wig : Sonic Advance or Sonic Advance 2 cartridge
Ulala hairstyle: Sonic Battle or Space Channel 5: Ulala’s Cosmic Attack cartridge
Unlock Hard Mode:
Beat the game on Normal Mode.