Don't know if it had been freakish luck or what but it seems like through most of these events I've had both drops before the event was halfway over. Only one so far has refused to drop throughout an entire event for me, Festival Bonny. (Don't have Sporty Jeanne from the current event yet, but we're only a day into that one so that one still has plenty of time.)

I think supposedly since the 2.0 update a few months ago they've had some sort of "pity counter" type of mechanic in place? (Meaning that something is supposed to increase the odds for the desired result each time it does not happen, so every time it doesn't drop a girl it should in theory be making it more likely for the next result to be a successful girl drop.) But all I really know about that is that there had apparently been an oversight in the system when world 9 released where some players got all 3 world 9 drops in their first 3 fights because they had spent the previous weeks farming for money on a boss from which they had already gotten all 3 drops so the system overcompensated when adjusting the odds.

Personally, outside of the event drops, the ones I'm still missing are Sung Tai Ki (world 3), Housemaid (world 4), Kelina (world 6), Leeditt (world 7), Marina (world 8), Edna (world 9), and Zoe (world 9).