Huh... it seems Mary actually is just a really really lonely kid and desperately want somebody to care and love her. I knew she was lonely from first chapter, but not to this degree.
On the other hand, I really feel bad for Colin. Mary seems to only see her as "friend" and that's only because she's convinced herself that no one but Colin care for her. Soon she will realize that it's not true, and that's probably where the author will begin to show the hellish part of the story.
For now though, this moment before the storm probably is really the happiest time period ever for Colin. Enough for her to endure all the pain and negative feeling, and feel truly happy. Colin truly look happy and innocent when she was going to meet Mary. I only hope that she won't ever think that Mary betrayed her, like her old snitch friend.