Microsoft has its Personal Vault on OneDrive, but if you prefer keeping your most private files on your device Google is now working on a similar feature for their Files app on Android.
The feature, called Safe Folder, is a PIN-protected folder on your handset which can only be accessed with the password, and which is invisible to other apps on your handset.
The data is encrypted with a 4 digit pin and Google promises that it is completely inaccessible without it, meaning if you forget your PIN the data is unrecoverable. The data is also lost if you delete and re-install the Files app.
The feature was discovered in a teardown of v1.0.315 of the Files app by XDA-Developers, and it is not known when it will be publicly released yet.
It is notable that a 4 digit PIN does not offer any significant protection, meaning Safe Folders may be enough to hide files from your wife but not the FBI.