There is a boy that follows Luna wherever she goes... Crossover between Rowling's Harry Potter and Pullman's His Dark Materials (and those are the rightfully owners of the characters in this story).
There is a boy that follows Luna wherever she goes. He does so unobtrusively, almost shyly, not wanting to be of any bother. Sometimes he walks very close to Luna, smiling at her. At other times he is far away and can hardly be seen at all as he is lost among all the colours that is the world. But he is always there, as he has always been and always will be.
Once he was very young, nothing more than a little child with a round face and big, surprised eyes. Now he is older, and even if the eyes are still big and surprised there is also a new wisdom in them. In fact, he looks quite a bit like Luna herself. But there is something strange with this boy. Even if Luna is full of colours, there seem to be none left for him. He doesn't mind, though.
Luna is glad for the company of the boy that follows her. Sometimes when she is sad and there is none to comfort her, the boy will walk close to her and smile, promising that everything will be for the best. Sometimes when Luna is lonely the boy will be close as well. Luna will talk to him, or show him a particularly interesting article in the Quibbler, or just sit and think in his company. The boy never says anything, but Luna can talk for both of them. She enjoys the company of her silent friend. He is kind and gentle and nice, and Luna tries to be nice in return. Strangely, the boy never seems to be sad or lonely. When Luna asks him about it and inquires what the boy himself would like to do, he only shrugs and smiles shyly.
But sometimes Luna forgets about him, especially now when she has real friends of colour that want her to spend time with them. Then the boy can be lost for long times before Luna remembers about him and goes looking for him. He is always close by, standing just out of sight, and he always comes when Luna asks him to. When that happens Luna apologizes, but the boy just smiles and shrugs and doesn't seem to mind. Luna feels bad about it though. No one should be forgotten and left without company. She has even invited the boy to come and play with her other friends, but the boy kindly refuses. He is Luna's friend, after all, so what would be the point in the company of others?
They have always been together. They always will be.
The boy is Luna's death. When Luna dies the boy will step forward and guide her to the land without any colours, the land of the dead. Luna knows this and she is thankful for having a friend with her on that last, scary journey, someone she knows and trust. She remembers that sad day when she was nine years old and her mother walked away from her with her death. Luna knows that when she dies her own death will take her to her mother. She will be waiting at the place where she was brought, and she will hug her and sing for her and laugh in the way only she can, just as she always used to do. Sometimes Luna can't wait for it to happen.