I feel like maybe Tsukasa is the one who gets her girlfriends so dependent on her that they threaten to commit suicide if she leaves, not the other way around.
My prediction? Tsukasa and the main girl get together, Tsukasa continues to isolate the mc and tell her that she needs her, and then it ends up just how it started, with Tsukasa chatting up some other girl on the phone about how clingy the mc is, and leaving her after making her dependent on her.