I notice that after getting all the girls from pearl purchase there are still some blank spaces in my CG collection. Are they something like secret characters?
The ones on the first page are seasonal bonuses. Play during March to get 2 CGs of Easter Fuck-Bunny Helen. (requires Helen and Amber at lv100+) Play during October to get 3 CGs of Sex Witch Helen. (requires Helen, Kathrine and Heidal at lv100+) Play during December to get 1 CG of Santa's Little Fuck-bucket Helen. (requires lv100+ Helen)
(Nicknames are just for fun.)
The 5 that are still empty after obtaining Delia are reserved for a Countryside girl who has not been implemented yet AFAIK, despite her pictures being included in the files.
Edit: After resetting for Pearls, I was able to discover Leanne in the City, not the Countryside. She is the one beneath Delia the Nun.