|I've only seen their relationship as friendship.
Spoiler (highlight to view)
The only romantic relations Reira has had is with Takumi, Yasu, and Shin.
Ren probably sleeps around (but in the manga, we only see him with Nana). In the end, I'm sure Ren only thinks of Reira as a close friend. He couldn't turn to Takumi about the drug addiction, because that guy has other things on his mind. The other Trapnest member (the blond one what's-his-name) probably wouldn't be too great of a support, so all he has is Reira. Plus, they're childhood friends.
I'm sure Yazawa planned to make Nana/Ren that destructive, yet indestructible couple, so I'm sure his heart (at least) is faithful to Nana.
And as for Reira, I'm quite sure she's fallen in love with Shin and just wants to hang onto Takumi for selfish reasons.
Last edited by In-tuition at 1:24 am, Dec 20