I have to say that I was truly enjoying the series up until his introduction.
I have such a great deal of hate for this character because I have such a predictable fear of what he will become.
To me, Nanatarou will become the major pushing force to keep Mizuki Female. And I hate him for his obviousness about it. From the very convenient fact that he's an estranged heir to a Lingerie Conglomerate, to the ability where he was instantly able to clarify Mizuki's gender and then proceed to blackmail and fall in love with him, as well as the great deal of Jealousy he was able to create immediately on his introduction.
He will easily become the character who helps Mizuki's hardship as a female, as well as pushes him to struggle between his view on which love interest he should pursue, and then be coaxed or thrown away or otherwise toyed with without having much story of his own. He is the incredibly sad existence of a Writer's Convenient and I just can't help but hate him.
To make a simple example (A likes B and C. A's driving force to like B is created by B and seems like the correct way to go, but the new interest of C is seen as weird and causes turmoil but B is still the major interest. D is introduced to make the relationship with C seem more viable and becomes the major driving force in AC shipping.)
He is the old man who teaches the hero a secret technique but the hero hasn't decided which faction to fight. He is the officer who points out the obvious while the detective figures out the case but struggles to point out the murderer as his friend.
He is Largely unnecessary and unneeded by both the story and the characters within the story and only serves as a shortcut to lengthen the drama while making the technical hardships disappear.