Since the beginning of Let the Right One In (orig.: Låt den rätte komma in), Eli was accompanied by Håkan, an older man, who tried (not very successfully, though) to kill people and gather their blood in order for Eli to not have to kill anyone, it seems. And when Eli visits him in the hospital (after he failed to kill another one and disfigured himself to prevent her being discovered) he finally sacrifices himself to Eli as blood source.
But I'm not sure what to make of his character. What was his relation to Eli and why did he help her, up to the point of sacrificing himself? While she said he was her father in the hospital, this doesn't really seem to match her relationship to him as depicted in the movie and to me seemed more like a cover-up just told to the nurse (but I might also be wrong with this evaluation).