N64 version
You can unlock a Cheat called 'Play as Deadsider!' then enable it in the secrets menu!I don't have any info on this sorry N64 version only!
N64 Location:[]
Go to the Asylum: Gateway and go to the path before the door on the left under the bridge. Follow it until finding 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.
PC and Dreamcast Locations:[]
Not in those versions, N64 only!
N64 version