While playing, press ` (tilde key) to enter the console, then type:

Code                         Result 
Invuln                       God Mode 
GimmieGimmieGimmie           All Weapons 
NoClip                       No Clipping Mode 
NoTarget                     Monster Targeting On/Off 
Spectator                    Fly Mode 
ChaseCam                     Chase Camera (5 Different Angles) 
CyclePlayerClass             Cycle Through 4 Player Classes 
NextArmor                    Increase Armor and Health 
PrevArmor                    Decrease Armor 
NextMonster                  Go to Next Monster 
PrevMonster                  Go to Previous Monster 
RestartLevel                 Restart Current Level 
Give [Itemname] [Amount]     Give Items (See List Below) 
CameraLock                   Unknown Effect 
Debug1                       Unknown Effect 
Debug2                       Unknown Effect 

     Note: Some of these may not work when typed in the console. There
is  another  more  reliable way to enable them. In the PROFILES folder
under the game's install folder are the player configuration files. In
whatever  profile  cheats  are  to  be enabled, open the .cfg file for
editing (example C:\KISS\PROFILES\PLAYER.CFG). Find the line:

"Developer" "0" 

and change it to: 

"Developer" "1" 

     Also, remember that the game will reset this to "0" when exiting,
so it will have to be changed often.

     Now,  in  the game, go into SETUP/KEYBOARD and scroll down to the
bottom.  There are two new sections titled "HUD TOGGLES" and "CHEATS".
Any of these settings can now be bound to keys and used at any time in
the game.

Give Items:

Fist
BeastClaws
Punisher
Thornblade 
Scourge
MagmaCannon
ZeroCannon
Windblade
Galaxion
SpiritLance
Stargaze
MagmaAmmo
AirAmmo
EarthAmmo
Health 
BloodRose
ChaoHeart
PickupTrigger
Key
Ticket
Hawkeyes
Gauntlets
CrystalBall
Twister
AngelCharm
Boots
Draco
RageOrb
WaterAmmo
Pamphlet
Belt
BlackDiamond