im not sure about any IFs where we can fall inlove with the Demon\voice in our head, But i do know two IFs where we can romance a demon\Spirit
1st - demo paranormal: a completed IF where you can romance a demon (Both Ro and mc are not customizable though, They are the authors ocs if im correct?) it can be found on Itch. io
2nd - Laid off: You are able to romance a spirit living in the apartment you just moved into, Both Mc and Ro(Though only by gender for the Ro) can be customized, There are other Human Ros too, It can be Found on COGDemos :) Though it is in early development and only chapter 1 is out currently
and if you were only looking for IFs where you can Romance a voice\demon in your head? I will look for some 😼