Board Thread:Questions and Answers/@comment-32556318-20170712153049/@comment-36009152-20200116152232

Ah, I see what you mean about the relation between biology and entities like what Yu is. With the possibility of Yu being 'born' it was mostly in response to this one:

Crimson lotus ichinose wrote: I really highly doubt that when Noya had killed him before then he had borned on biological way and from biological parents!! I'm a writer and a medical student so I know about medicain more than anybody else and so I can say that without any doubts that Yuu CAN'T born in BIOLOGICAL way AT ALL Yu may be a created being, that black substance, a demon, seraph/whichever aspects he is. What I was thinking is that a being like Yu may not be human but could be implanted into Yu's mother and born that way. Asuramaru knows the present day Yu best and said that a part of him is not human (so he is part human), so maybe the present day Yu is human, born as one, and born with the seraph gene that way.

The past Yu may also have been a regular human orphan Sika Madu found, just like Ashera, but subject to experiments which altered him into what he is. In that case the past Yu may still have been born with the gene as well.

The main points are that whether he is a human or not, Yu could have still been born to his mother (the Yu that Noya murdered was smaller than the one Ashera saw so there are different aged Yu's) so she saw him as her son until she turned against him. The other point: there's not really anything to 'prove', all we can do is examine what we are shown in the story to try and shed some light on the question as if we were explaining calmly to others to help them understand a perspective.