After the war Charlie runs away (again) because he doesn't know this new family or this new world any better than he knew the old one. He runs because he wasn't any use during the war and because Percy's home now so he can be a lost boy again.
He runs because he doesn't know how to be brave like his brothers and sisters and because he's lost his little brother and his best friend.
He runs because the world is changing and he's still stuck at seventeen years old and a need for fire and empty rooms.
(He runs because when he left home there were nine of them and now there are only eight and Harry, Fleur and Hermione.)
So when he comes back to Britain, sparse and scarce and still pretending to be a grown up, he always gets surprised at how much his family has grown. (Because by now there's Teddy and Victoire and Molly 2 and another baby growing in Fleur)
So when he comes back to the Burrow and meets the children he's a little (a lot) surprised at how much they feel like family. And then there's Teddy who isn't really except -
Except he's seven years old and Charlie can remember his mother at that age and there's a little boy standing in the garden, laughing at the gnomes and he's got purple hair that shines in the sun and -
for a second -
But Charlie is an adult now and Tonks is dead and this is her son and – the hair is blue now and Charlie is crying.