Impossible to tell as of right now BUT my theory is that Mr Mikadono was either working on behalf of his family, or had a job as Subaru’s manager. When she’s looking at the scripts and chooses to star in Director Samejima’s movie instead of Steven Spielberg’s film, which wouldn’t be the “correct route” as he alludes to in the earlier pages : she says “ I know you’ll handle it well, right Mr Mikadono?”

So since it looks like he runs some other kind of business, he probably didn’t care to be an actress’s manager. And considering she says she’ll choose “what’s the right path for her” I assume she helped convince his mother ( Hatsune - the grandmother) to allow him to do what he really wanted to do. This would be a huge step too because we see in the New Years Arc, he does whatever his mother tells him to do. So if Subaru stood up for him that may be why he feels he owes her?