What's the logic behind naked card drops? From what I've gathered the card needs to be above certain rank and present at your current team, but I have no idea what are the requirements beyond that.
When a female character with a nude card reaches rank 40 (****) her new nude card becomes a high priority drop, similar to the Kessel maids. So by managing which of your characters are above or below rank 40, you can "force" the game to drop a specific nude character the next time you open a chest that can drop that nude character card.
Some characters also need to have some or all of their related character cards in your collection to drop their nude card. For example, I couldn't get Sioux nude card to drop since she requires Haurein and I didn't have Haurein. A better example is nude Yuki, who needs nude La Saizel to unlock.
Some characters' nude cards have priority over others. Another factor is that the longer the character has been over rank 40, the higher priority their nude card is. So it's important to always take new nude cards since they will block other characters' nude cards if you don't take them.
In my example, since I leveled Pope to rank 52 for the Warg fight, her nude card blocked other Free Cities and Light character nude cards from dropping. Even though I already had nude Pope from my first playthrough, if I don't take nude Pope she will block other characters' new nude cards from dropping. If you look at the pic above, I got nude Tillday last since I had to go back and remove nude Pope from blocking her card drop first.
Each chest on a map has 3 slots, and each slot will drop a fixed type of card, with different thresholds for what kind of card it will drop. For example, one slot will always drop Zeth cards, another will always drop Fire cards, etc... depending on your treasure luck threshold, the kind of card ranges from dupe generic (lowest) to new secret rare (highest). So if you manage your team so that all of your female characters that you don't have nude cards are below level 40, if you find a chest that can drop nude cards (i.e.. it has a low threshold for the type of card you want to drop), then if you go back to the previous phase and give a meal ticket to the character you want the nude card to drop (so she is the only new character to reach rank 40), when you open that chest her nude card will be in it, assuming it doesn't have other requirements and there's no other nude card with higher priority blocking it.
Basically, when a character reaches a new rank threshold (10, 20, 40, 80) that character will change the priority of card drops, changing which cards are inside chests.
If you find a chest that can drop 3 secret rares (I used the turn 15 free chests map chest), the new nude card will replace one of the secret rares.
In other words, if you find a chest that dropped a nude card, it will be able to drop other nude cards of the same type, so you can use that chest to get any nude card of that type.
Secret rares are similar, i.e. if you want to get Secret Rare Gandhi, you have to level him to rank 40 first.
