Natsuki is Yone-sempai, his name is Yonezaka Natsuki ( http://www.mangago.zone/read-manga/kataomoi_to_parade/mf/v01/c001/15/ )
And in Vol 2 Chapter 11 we don't see the face of his lover !It's Yono-sempai who sneezes ( http://www.mangago.zone/read-manga/kataomoi_to_parade/mf/sa-v2c011/pg-11/ ) :)
And he is got some kinda ring like promise ring on his finger
Yes, his lover would be Ao and hence the OP's question which still remains unanswered, strangely....
I’m positive it is Ao, I mean it’d be crazy to do 3 separate chapters with Ao for it NOT to be him?? With the “Whoa, you okay Natsuki?” keep in mind that is the translators interpretation of whatever it was in Japanese (Japanese is not really a straightforward language so the English translations can vary depending on the translator). Also, that line might’ve been translated before they found out about Ao’s character.
Another note, “Whoa, you okay Natsuki?” had no exclamation mark. I personally read it in a kind of deadpan manner. So (to me at least) it didn’t seem far from what Ao would say.
Lastly, in the author’s note of Natsuki’s story Act 2, sensei wrote “..let’s draw the bits and pieces leading to that Yone with the ring in the very last part of the main volume..” but then she did say “..if they actually get together, I’ll be really happy..” like she doesn’t know if they'll actually get together.. ahaha. Anyway, I just hope it's Ao.
Sorry! This reply is way too long for such a simple question. I’m definitely looking too much into it, but I wanted to answer your question properly because I wasn’t pleased with any of the answers you received so far haha.
Thank you!!! I always skip the TN notes HAHAHAH but what you said sounds about right.
Really weird because in the last chapters where Yone-senpai was wearing a ring, Yone-senpai's "lover" seemed to be a different person. Ao looks like someone who'd have a monotonous and indifferent manner of speaking. I wouldn't peg him to say "Woah, you okay Natsuki?" (Vol. 2 Chapter 11) Or maybe I'm just overthinking things?