There's a lot of them you can't ask for sex since the last update. Basically to get hers you just need to do the main story route / quests.
Jeeze....Ash's raction when I said no to her confession really made my heart go out to her. I wouldn'tve thought a game like this could hit you so hard emotionally.
Take a look at your panty colection.
If you mean the costume for the lady in the lake, just go talk to everyone and askabout the costume.