The following Gameshark codes will allow you to play as the 2 Player Characters in 1P mode. Note that there is no way to play as the Dark Chao walker, the game freezed when loading any level.

PAL VERSION

Activator Code (Must be on for the codes to work)
K1Y7-REN2-TTCF9
QRY5-P305-CMKY6

Play As Amy Instead of Sonic
QWJ0-TN29-RW4U2
4D1N-WXEQ-BBQWZ

Play as Metal instead of Sonic
Z535-4DKM-V2GDP
AQ06-WJZM-TBMTE

Amy/Metal Fix (MUST BE USED TO PLAY AS AMY OR METAL SONIC)
D8F8-9N9W-TPE11
K3HZ-29ZK-C64FJ
KK37-NVP9-MXU2D

Play As Tikal Instead of Knuckles
7G9D-U7DR-MYV5R
ZBNB-UM4D-534H6

Play As Chaos 0 Instead of Rouge
NHW6-G645-XTU6Y
ZNQQ-32XU-TRMUF

Play As Chao Walker Instead of Tails
87VT-Q0B9-3CYVM
1233-510M-ZUD39
N59Z-NBDZ-7Z2ZA

NTSC VERSION

Alternate Char Models Enable
RYMJ-CCZX-687EM
1PFB-GG53-T1N1E

Play as Amy (Requires Alternate Char Model enable)
RGN8-UB1X-GM602
ZY4D-77VC-U82FJ

Play as Metal Sonic (Requires Alt. Char. Model enable)
E8RM-NAJW-YH0Y6
GXJR-A0DF-HKJB9

Amy/Metal Sonic Fix (Required in order to play as Amy or Metal Sonic)
7Z3H-GG4Q-8BDQ0
Z445-FZYW-UJY0V
Y2QZ-V8TX-HJ32P

Play as Chaos 0 (Requires Alt. Char. Model enable)
XW33-V812-GRBYC
16PT-F0Y2-55A8W

Play as Tikal (Requires Alt. Char. Model enable)
M4RD-RV0P-8HXM0
QFDD-VA7Q-3ZBWA

Chao Walker (Requires Alt. Char. Model enable)
HK3Z-PAFP-1HF6W
KK7P-67T3-NY1YV
0G00-W2FG-DNYE1