If you beat a certain time in the Time Attack Mode, 
you will unlock a ghost which is very hard to beat.
Here are the times: 

Mute City          1:23:000
Silence            1:13:000
Sand Ocean         1:15:550
Devil's Forset     1:23:000
Big Blue           1:35:000
Port Town          1:36:000
Sector a           1:22:000
Red Canyon         1:19:000
Devil's Forest    21:24:200
Mute City         21:19:000
Big Blue          21:07:000
White Land         1:40:000
Fire Field         1:23:000
Silence           21:35:000
Sector            B1:45:000
Red Canyon        21:40:000
White Land        21:13:000
Mute City         31:46:000
Rainbow Road       2:11:000
Devil's Forest    31:20:000
Space Plant        2:04:000
Sand Ocean         1:35:000
Port Town         21:52:700
Big Hand           2:27:000