There is something intimate about the way they call each other by their surnames in front of others, but by their first names whenever they are alone, just like during their high school days. That's no something you often see in j-dramas.
Even when they reunited after all those years, the first words they exchanged were their first names.
Their relationship had started off as an academic rivalry and grew into frenemies-like friendship, where each interaction centres around a challenge, but it's clear their relationship stopped being a mere rivalry a long time ago and has grown into something deeper, more messy, harder to define and, frankly, ineffable. While those challenges and bets have gradually become a guise to prolong their connection and not to lose each other.
Even Zen, who, unlike Ryo, doesn't fully realise or comprehend his true feelings towards his long-time frenemy, can see that, calling it a habit.
Ryo, on the other hand, is much more aware that what he feels for Zen goes well beyond what one feels for a rival or even a friend and uses his victories to get himself these little dates with Zen so he could spend more time with him (his main motivation for keeping the competition going beside it serving as a pretext to prolong their relationship), to which the object of his desires clearly remains utterly oblivious.
I mentioned that FFL is the first BL drama since Utsukushii Kare that I've become obsessed with, and one of the reasons might be that in many ways it reminds me of UK. Whether it's the high-quality of production and acting performances that set it apart from other BL dramas, but mainly, it's the narrative similarties - the muddy high school situationship, the separation and reunion afterwards, both men pining for each other but fearing rejection. And most importantly, just like UK, there are all these little moments and details, a fleeting hidden glance here and there, an off-handed comment that lets slip their true feelings and reveal the truth. Just like when Ryo comes to open the door for Zen.
It lasts seconds and there are not even any words spoken, but through the glass in the door, before he opens it, you can see his complicated expression, like he is holding his breath, all nervous and insecure, and later the happy smile his boyfriend came to his home, but also the cocky bravado he puts on next to mask his true feelings.
