You don't need to search the internet or download any other mod to hide helmets from the character. All you need is a simple browser add-on that allows you to hide items.

The item that is being hidden is placed in the cloak slot, which can be seen on the paperdoll. When the character is in their inventory view, they can see the hidden item, but it will be hidden from the other characters.

To use this, you need to download a browser add-on and install it on your computer.

This add-on will allow you to hide items from your character. It works in both single-player and multiplayer.