Shadowman
  
Dreamcast

Deadside Shotguns:


  Go to the Wasteland and enter the Temple Of Life. Go to location with the Baton 
  and go to the altars. Keep pressing B at different altars until the "Secret 
  Activated" message appears. Go to the Temple Of Life after you get to the Baton. 
  Warp out of the temple, then warp back in. Go to the first altar on the right 
  and press B when you have less than 100 Cadeux. Then, go to the next one on 
  the left and the "Secret Activated" message should appear. 
Flame on Mode:


  Go to the Lizard Kink A.K.A. Doctor Death, in a long hall with a locked gate. 
  The "Secret Activated" message will appear when you enter. Go to Dr. Death's 
  Schism Gate and go to where you fight the helicopter and destroy it. Then, go 
  to the tunnel to the right of the entrance. The "Secret Activated" message will 
  appear. 
Pea Soup Mode:


  Go to the Temple Of Fire. Locate the pool of blood and find the door that leads 
  to a room with a ramp going up the left, which leads to another ramp. Move up 
  the ramp and look to the left to see a hallway. Jump on the railing and jump 
  towards the hallway. You should grab the ledge in front of it. Pull yourself 
  up. The "Secret Activated" message will appear. 



Disco Mode:


  Go to the Cathedral Of Pain and drop down the shaft into the Marcher lava. Go 
  on the top of the spinning blades, then find your way to another shaft that 
  goes down. Drop down this shaft and the "Secret Activated" message will appear 
  as you fall.
Shotgun Head Mode: Go to Temple Of Prophecy (Marcher) and enter the 
  "Flambeau maze". Find your way to the center of the room on the ground level 
  and the "Secret Activated" message will appear. 
Big Head Mode: Warp to Asylum: Playrooms. Go to where the playroom cells 
  are found (with a pool table and a playpen). Enter one of the empty cells and 
  wonder around until the "Secret Activated" message appears. 
Invisible Man:


  Go to London Underground via Jack The Ripper. Go to the middle stall in the 
  women's bathroom and the "Secret Activated" message will appear.



Play as a Dog: 


  Go to the Asylum: Gateway and go to the path before the door on the left under 
  the bridge. Follow it until you find a room with a Govi and some boxes. Climb 
  the boxes and jump on the rafters. Jump to the second rafter, then jump to the 
  third rafter. The "Secret Activated" message will appear. 
Play as Bloodshot: 


  Go to the Temple Of Blood (Nager) and enter the small bonus room reached from 
  the platform that is knocked down by the large hammer. The "Secret Activated" 
  message will appear when you walk in the room.
Play as Deadwing: 


  Go to the maximum security cell in the Gardelle county jail. The cell is the 
  one farthest to the right, on the right.


  The "Secret Activated" message will appear when you walk in the room. 
Play as Nettie: 


  Go to the Mordant St. NY. Follow the paths until you reach a room with a lever. 
  Pull the lever and the lights will turn on. Enter the elevator and go up. Go 
  find the door, then look for a brown patch on the floor. Jump as far as possible, 
  then jump again. If you are quick enough you will get over and the "Secret Activated" 
  message will appear. If you fall through, use the bear to warp there and try 
  again. 
 
Thanks to Revolution reader Dragon Child!