How can he still think she is Natsuki when he already know she is rich and she event told him she investigated him. Even if all that 20 mln borrow stuff was planned by his father, why would he use her(a girl from some big company) as the girl for his plan.
I think that all that borrow 20mln thing couldn't be his father plan. Natsuki behaviour didn't look like "playing" and she even went so far as to giving him her first(it failed though), all that crying and talking about not being suitable for him is either his father telling her to cut ties with his son or she feeling guilty after what she made, still we need explanations. What happened with that 20 mln, what was the real purpose of it.