Unless they changed a few things, The Fool arcana grows as the story progresses so there is no need to be careful about your dialogue options with them.

As for Yukari's Social Link The Lovers, you need to get to the end of July I think it is, could be August, before it becomes available. All the girls on your team require you to max out one of your social stats (Charm, Intelligence, and Courage) so for Yukari it's Charm. Her social link is interesting but you have to be careful cause she has some interesting mood swings and sometimes the best action to take in her SL is not the one that seems the most obvious.