Promise?

Dean was a kid when he had found out that an angel, Castiel, was watching over him. Castiel and Dean were the same age, and they basically grew up together. They even had the same birth dates. Albeit, only Dean could see him, and was very sure that Castiel was just an imagination of his, he didn't care. Castiel made Dean happy.

But what happens when Dean finds himself starting to fall in love with Castiel... A mere imagination of his?