Do not take the "Red Stone" without "Letter for Claudia".
Best Ending
Bring "Letter for Claudia" and do not take the "Red Stone".
Normal Ending
Bring "Letter for Claudia" and take the "Red Stone".
Syringes
Syringes are useful to stabilize your heart rate. When you "free" any ghost, you can go to the android's room in the main hall (with the moon monument). The android's room is between the Energy Area and the Work Area. In the desk, behind the android, you can find a syringe. When you free another ghost, you can find another syringe. Note: You can see that the freed ghosts are painted in the room's walls.
Worst Ending
Take the "Red Stone" without "Letter for Claudia".