If you’re just curious about who the girls are,

Once you get all of the fairy wings, you unlock two new girls (the nymphojinn) named Moxie and Jewn. They are aliens, kind of like Celeste, but more magical-looking

If you want to know the story as well,

You have to do 4 consecutive dates with mind-controlled versions of the girls. I’m not sure if the pairings are random, but the date gifts they have equipped are the same. After you finish all 4, you do a bonus round against Moxie and Jewn, after which you get their photos and unlock alpha mode and non-stop date

Obligatory “typed on mobile” disclaimer, you know the drill here. I am also not sure if these are the only secret girls, but these are the only two I’ve found so far.