The tension in this book from what for me felt like a shoehorned character who bumps into Nana and seems to "instantly" fall for her after forcing her to go on random "dates" with him, wasn't that enjoyable. It did feel very forced. I liked the idea of someone else seeing Nana and vying for affection/attention, but I think I would've liked it more if it wasn't a stranger.
Tendo needs to know and realize that somebody else could potentially be a better match for her than he is, he wasn't very kind to her for a good while, and he's only recently decided to show her in a more obvious way how he feels about her. 

All that said, when he was challenged, he did step up to the plate to show Nana and others that he cares for her and wants to be with her. He should have done that months ago. Also, finally got a confirmation of their ages, he's 31 and she's 24.  I don't know if it was mentioned earlier in the series and I missed it or this was the first time their ages were stated. It is an age gap, but it's an age gap where they were both always adults. When they had their chance meeting 6 years before the start of the main story and in the present. And that's the only way age gaps don't ick me out.