Okay so like, honestly the transfer student is the least interesting character to me in this series by a long shot right now. She's just being honest with the MC and kinda clinging to him while not really forcing the issue beyond occasional reminders of how she feels. Meanwhile, he's resistant to her advances, and has made his lack of immediate interest clear, so there's no momentum on her side yet. Like sure, fmc being straightforward is refreshing for us manga readers, but only that. It's not enough on its own to make it interesting. There's no tension, and at least thus far, chemistry has yet to really be established either. I don't know how interested I'd be in reading a series about just the two of them, at least with the current setup.
BUT, I also feel like she's the most important factor making this series work -- her being straightforward is the gas on the fire that pushes the other girls out of their comfort zones and propels them to action. She makes for an amazing foil to them (as well as the MC himself) which serves to make them shine under fire and therefore drives the series' momentum.
I think this is why this series is so entertaining so far; instead of a harem where nothing ever happens because nobody wants to make a move, the characters have a fire lit under them which shakes things up and forces action, which in turn becomes our entertainment and gives us reasons to root for them. Of course, nanao being refreshingly straightforward and winning the small battles by default in the face of such opposition that she's hardly even aware of gives you reasons to root for her, as well. So everyone can still have a horse in the race, thanks to this sort of balancing act her presence enables. She wouldn't be so interesting without them, nor would they without her. This is really demonstrated expertly in chapter 1, where every heroine got their own dedicated title call -- this author understands that each of them is vital in order to make this setup work.
My only fear at that point is that, after a certain point, this can basically only end up turning into a "obvious losing heroines flail in a panic and decide to take action because of unexpected element, but still crash and burn in the end anyway" kinda progression..