I believe this will most likely stay platonic, why?
The two main characters are both very lonely individuals whom have most likely never experienced any kind of real love or company throughout their lives, so when they finally meet someone who makes them feel like they belong in the world, somebody whom they feel they can express their true feelings around, they feel obligated to protect them. Perhaps this is because they feel like they have to give back somehow, but because they lack much human interaction (Nagisa) and aren't very confident about themselves (Ai) they fear that the other person may view them as a burden.
I don't think this necesarily points to romance, but hey! I could be entirely wrong, and maybe there will be a romance between the two, because as somebody else pointed out already, the whole acknowledgement about the other's body is often associated to falling in love, which wow that would be really fucking weird because thats literally her sisters underage stepson but this is manga so nothing fazes me. I just hope the comforting aspect of this manga remains, and despite what some people may say about this series, I'm really glad that it's here.