What data is accessible after resetting administrator password with FileVault enabled? I just broke into my boyfriend laptop and reset his password. I'm afraid that he do the same to my laptop.

Will he gain access to my notes and browser bookmarks if he resets my password and FileVault enabled?
If FileVault is enabled it is impossible for anyone without the FileVault password to access the data on your computer including any services like Notes which you may have.

Please note that what you have just done on your partner's laptop may be illegal and I would caution you that this is a public website where you may just have admitted to a crime depending on your location.