Secrets is what kept them together.

Though Allison, Spencer, Aria, Emily, and Hannah were all the best of friends, they all kept their deepest secrets from each other. The only one that knew was Allison, and she thought having secrets from each other was a good thing. She would always say that their secrets is what kept them together. Unfortunately, Allison would use their secrets to threaten them. The odd thing is Allison's biggest secret was kept between herself and her mysterious, older boyfriend that none of the girls knew. She never told them who that older boy was, and they never figured it out either.