Jessica was actually closer to Yoona before debut and in the beginning of debut... but somehow, when Jessica decided to warm up a bit, she got used to the personalities of those like Yuri. Though I personally believe that because of Yuri's outgoing and I don't care spirit, Don Jess saw this as an awesome candidate for someone who would do her evil biddings. Jk. Yuri actually got close to Jessica because of Jessica's friendship with Yoona... Before that, Yuri and Jess were just meh towards each other.
Yuri (I'll admit) is in a love triangle with Tiffany and Jessica and I think that's where she belongs if I don't let my bias get in the way. However, YoonYul beats YulTi (just by a little!) and Yulsic in my book (even if SooNa is freaking cute). Man, that YoonYul pic in the Video came out of nowhere and I LOL'd
-O_sLaVe