@janedarling Sorry it took me 5months to see your reply, real life happened and I forgot about mangas.
I don't remember much of the story but whenever we see the young Hirotaka when he was in school with Narumi and how much he treasures those days, it feels like that's when he started liking her.
Then when Nao starts talking about his brother crush (and Nao wouldn't consider a videogame character), I'm pretty sure he meant Hirotaka finally starting to date Narumi.