But who is that girl

I met this girl.
I caught her hiding in a corner.
Just who is she.
She stood quietly by the side watching everyone else.
She stood by that boy watching over him.
She made sure that he was happy.
She made sure that he got everything he wanted.
She listened as that boy talked.
She talked when that boy kept quiet.
She hung around when that boy was troubled.
And left when she had troubles.
She cried when that boy cried.
She laughed when that boy laughed.
She knew that boy didn't need her.
But she would be right there when he does.
I watched her as a bystander.
And i watched as she left with a broken heart.