which crystal is it to reset?
Hopefully, you've figured it out by now, but if not, or for others:
1. It is not the end of the game you have to reach. Clearing Chapter 1 will allow you to access the crystals.
2. The crystal on the bottom right which says "LV," followed by four kanji, followed by "2" (Like so: LV####2) is the one you need to activate. It may reset more than just Yuna, but it won't reset Tifa, and I didn't spot any alternatives.
Note, also, that if you do not raise her libido, Yuna will still rebuff attempts to grab her. I assume some stat contest (and RNG) is involved in order for the enemy to succeed at that, but with a high enough libido, the girls simply let themselves be grabbed. (400+ probably guarantees a grab, though I have no idea if anything lower will also do so.)
The libido item is purchased from the shop in Midgar (futuristic city) with the suggestive picture over it. There's one that raises libido by 30 and one which lowers by the same amount. The descriptions have a "30" in them... so, go from there.
Another note, as far as I can tell, Yuna and Lightning can't get pregnant from random enemies (tested with the +pregnancy chance item and several vaginal cumshots. Also, the cg sets I've found online don't have any pregnant versions of the normal standing pictures for them).
Also, just to throw in an FYI about Lightning, she has a similar protection mechanic in Chapter 1, but Chapter 2 is entirely Lightning (it's also super short, and more of an extended event or short story, nothing like Chapter 1) and, as of Chapter 3, she no longer has that protection.
