Well, i spent 1000 kobans in last event, no luck, no girl from bosses. Right now, im fighting Dark Lord every day for his Rare girl, still no luck.
Bremen still has all his girls, even though he is already at his max level. Finalmecia also still keeps all her girls.
I really wonder if its only RNG (with 0.001% chance???) or there is more behind it (player's level || top quest || total number of girls || total money spent || ...)