So was naoji in a romantic relationship with the grandpa as well?

I don't think so. I think they were more like long term friends.

I really do hope so, coz that would be a little bit weird.

Not just a little, it would be so fucking weird

they don't, it was a friend that gave him home, a comfort zone and that's what made naoji lonely, he lost a person who gave him home. and he said it too, never drank the gramps blood and never knew the effects of it so it connects to he never had that relationship with the gramps nor anyone, i think.