Clue is in the nameWell, haku is a river spirit dragon thing, and he saved her from drowning when she was little. It's not entirely clear, but I think he is, in a sense, the river itself. Or the spirit of it. or whatever.
Clue is in the name
That said, I don't understrand the reference to river of her childhood days.
Have you SEEN THE FILM?
It even literally says in the film she remembers him from her childhood days.
No, she is already a child, so "childhood days" is nonsensical!
NoCube?
NoAnyway.. my guess is:
BIG BROTHER!
Really.Steel container is a bit vague