You can unlock a Cheat called 'Play as Nettie' then enable it in the secrets menu! Note: you will climb slower.
Not availible on the N64
Go to the Mordant St. NY. Follow the paths until reaching 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 fall through, use the bear to warp there and try again you need the floor that falls for the second jump.
PC and Remastered Location:
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" will appear just walk around until you find it.