[kujoe]

Theory? Occam's razor... It's actually very simple.

Luke is the son of Duke Fabre who had been kidnapped years ago. Upon his return, they discovered that he has no memories of his ordeal or of his life, from which he had to relearn everything... about himself, about his life and the people he's supposed to know. Others have noted how different he is now due to this, and he has been confined in the manor for years out of fear for another incident. Natalia in particular wants Luke to remember his words of proposal that he gave to her from long ago, and she tries to help him out.

And then, we have Asch who exactly resembles Luke, and who ambiguously looks towards Natalia's direction for some reason. Now something is missing in between, don't you think?

Here's another clue: other than Luke and Asch, Ion and Sync share the same voice actress.