1 You get it after leaving the mansion
The helmet key is among the elements you can't get in the beginning of the game. You will have to leave the mansion first until you reach the residence area where you will be able to find it there.
2 Get the 003 room key
You will need to get the key to room 003 inside the residence in order to get the helmet key.
3 Go to room 003
Go to room 003 which is located inside the Gallery room. Note that you will need the Gallery room key in order to reach room 003.
4 Complete the books puzzle
Complete the books puzzle inside room 003 by just arranging the books to get a meaningful image of a lady.
5 A hidden door will show up
After you complete the books puzzle a hidden door will show up. Go through that door. You must be prepared for a fight for there is a boss behind that door.
6 Defeat the giant plant
After getting through that door you will have to defeat the giant plant inside before you can get the key.
7 Investigate the fireplace
After getting rid of the plant investigate the fireplace to find a key lying inside. Examine the key and you will find that it says helmet key.