Castiel believes that Luna Lovegood does not belong to the earthly realm. She transcends any sort of brilliance humans can achieve, as if she was supposed to be something more, something heavenly, but then someone made a mistake. But Castiel knows that the Lord does not make mistakes, so he watches her from the beyond, wanting to know why his Father left someone so divine on earth. There must be a reason for her presence among the mortals.
Luna walks through the world with unshaken belief that there is always more to life. She believes in Crumple-Horned Snorlacks, Heliopaths, and Nargles. She watches for Blibbering Humdingers and Aquavirius Maggots. She reaches for things that cannot be seen and she lives straddled over the line between life and death. She can see angels and demons without being blinded by their grace, whether pure or fallen. She converses with them all, pleasantly unaccompanied by fear.
But she does not believe in any sort of God.
"We exist in a chaotic universe, Angel of Thursdays," she tells him, her tone dreamy and pleased. Her eyes shine with the otherworldliness of the full moon as she speaks, not human, but something so much more. "And our lives are essentially purposeless. They are our own, of course, but there is no sort of God to reign high over us all, no ultimate meaning for which we have been placed on this earth."
It hurts to hear her talk like this, because Castiel loves his Father, and he knows that his Father loves earth and all of it's creatures. For someone as special as Luna —who can see this world and beyond it— to deny His existence brings about a pain that he didn't realize angels could experience. He wants to prove to her that He exists, wants to bring her up beyond Gates of Heaven and present her to his Father, let her see that which she has blinded herself to.
But she distracts him, her laughing, "My Angel of Thursdays," (not Castiel, never Castiel), her bright smiles and silver eyes that look past what is there into the limitless beyond. His grace surrounds her life and he can breathe in her love and light, look through her eyes and see so much more and he wonders if she has managed to transcend not only humans, but angels as well. Because when he encompasses her being, he feels greater, stronger, more exceptional. It is not a feeling angels should covet, but he does anyways, bathing in the joy she brings to his sentience.
Yet, as all mortals are prone to do, she grows old and frail. She never stops looking, never stops believing, but she loses the strength to move, the capability of speech, and her silver eyes dim to grey when her eyesight fails her. She can still see, but she is dying, and her tales of the life beyond are dying with her. Castiel flutters about her with indecision, and he realizes that he has been corrupted by her essence, because angels should not be so concerned with a single human life. But in this moment he firmly does not care as he forsakes what he was created with knowing, and tries to breathe life back into her fleshly body.
She takes a shuddering breath and Castiel feels something he is hesitant to label as hope, because while he knows what hope is and how it inspires mortals, it is not a feeling that an angel should have. But as her eyes open, still grey and dim, their silver shine long gone, he feels something break inside him.
"Don't worry, my beautiful Angel of Thursdays. Merry meet and merry part and merry meet again. I will find you again."