Sadly, it is not actually possible to truly protect a profile on your computer once someone has access to your computer account. A master password (which is what you are asking for) is a great way to accidentally lock yourself out of your own profile, but it does not stop someone else gaining access to it. (Other products that used that approach caused this problem all the time, and there were numerous complaints about locked profiles causing data loss.)
As a very simple example, they could replace the application shortcut with one that shows a similar looking password prompt, and when you fill it in, they can send it to themselves. They could also copy your profile files, and hammer it until they get it. They could key-log while you type it, and send it to themselves. There are a million other things they can do, these are just basic examples of why it does not actually work.
If you don't trust someone completely, you should not be giving them access to your computer account. Ever.
This feature request has been asked and answered before here:
https://forum.vivaldi.net/topic/24238/master-password-protected-mode-to-protect-your-profile
