Hence the title of the game. "Our Red String" is referencing the red string of fate, from various Asian cultures, that ties Ian and Lena together, and how their string(s) is weaved in and tangled with the strings of all of their friends.They're both the protagonists, so yeah they're obviously the main focus of the storyline. I just don't think Eva ever intended them not to have the option never to get together as a couple. Whether they do or they don't, both their stories are inextricably entwined, and they end up in some form of relationship, even if it's just as friends.
I've written on it a long time ago, I think around chapter 8 at this point, but to quite literally quote myself.
The title is a reference to the Red String of Fate, an ancient Chinese or Japanese (I don't remember which) belief that people who are destined to find love and marry each other are bound by an invisible red cord. The cord can become tangled and bind with other cords, but will never break, so these cords can create a great web of fate at times. Hence why all these characters lives are interconnected and they seemingly keep running into each other even when they don't plan too.
We're seeing a snapshot of Lena and Ian's lives. Perhaps they have a red cord that binds them, or perhaps their red cords just got tangled with each other the night they met in that ally. How we play them will likely determine that, but for now they are bound by fate to at least be near each other.