You are a young angel, one who is widely different from the others. You allow yourself to feel emotions and follow your instincts. You also grew to have a crush on Castiel, but hide it quite well. You often reside on Earth with your charge, who helped you get a vessel for yourself.
You are walking back to the apartment when you sense Castiel's grace, which is strange because he usually sticks with the Winchester brothers. But then you sense his grace fluctuating and instantly hurry to the apartment. The only time when an angel's grace fluctuates is when they are entering a rut or heat. If Castiel catches you, then you will belong to him for all eternity.
Castiel hunts for you now. He won't give up until his rut is over or until he has claimed you. He had known that you would always bring him small gifts whenever his rut hits him when in heaven, which makes it easier to find HIS small mate. Your scent and grace are burned into his memory. You are his.