I agree 100% that's the only way one could have his vampy traits while also looking similar to Ellie. Especially since everyone who knew Ellie thinks One is him.
also if you think about it, Elli disappeared a few years ago One who's up a few years later bearing striking resemblance. Coincident? I think NOT! sorry Im rambling( ̄∇ ̄")
And finally I want them to have sex too ε=ε=(ノ≧∇≦)ノ ლ(´ڡ`ლ)
I agree. In Shell's memories, Ellie tells him a miracle has occurred. So if a vampire were to get pregnant by a human, I'd say that was a miracle. And the pendant was a snake and a flower. The snake is from Bones' family, and Shell smells like flowers. Coincidence? I think not.
So here are a couple more to add to your theory: What if Ellie was this human resistance leader? White something?
What if Bones is the one behind sending the Art to Shell? I mean, the same Jack who brought Cinderella is the one who was handling Humpty Dumpty. Coincidence? Bah, no such thing!
What if Pain really does have a soft spot for humans, just like his son. Maybe not to the point he wants them to be his master, but both Bones and Shell's mom made comments that made me believe that maybe they are more alike than we thought. And maybe Pain was actually in love with Ellie?
And one last one....what if the age of darkness, the little ice age, is about to come to an end? Shell's mom mentioned that it feels hotter in the south, even though it's midwinter.
This is so good. I can't wait for more, but then I feel bad because how can this have a happy ending for our two lovebirds?
oh yeah, and I am also eagerly anticipating for the foreplay to end (all of this so far has been foreplay lol). I think it will be soon. And Jack needs to be staked and baked (I am still ticked by what he did to poor Cinderella)

I think One is the son of Ellie and blue eyed daughter of the Bones. He could not be Ellie because Ellie is an Art (a human). But Ellie could be his dad because they share the same face and hair features. While One has some vampire traits such as the pheromones and blue eyes which he might have inherited from his mother who is a vampire (if my theory is right before).
Also, the man in the memories of One might be Ellie, his dad. And the pendant (with the snake and rose) which was given by the man might be from his mother (the blue-eyed daughter of the Bones.
I also noticed that there were men in hoodies in One's memories and there were blood, so I am assuming that some vampires (who might be hired by the Bones) pursued Ellie (his dad) and One and tried to kill him too.
What do you think guys? Oh I am so loving Shell and One too much! I want to see them make love soon, kyaa~!!