Well Tsukasa is a cheater so already it's a mega red flag. But just imagine this series will not make Tsukasa bad things to Nana, and it's just her, by herself who will fall in the chaos of isolement, dependance, paranoia and despair.
She was so cheerful and loving at the chapter 1 so explore this selfdestruction fall can be very interesting, to me at least.
But again, Tsukasa is a cheater so it's hard to believe she will not be implied in the toxicity of this series.