Leila’s presence functions as this unsettling ghost from Christian’s past that the main plot literally orbits around. She isn't just an ex; she’s a walking embodiment of his failed attempts at a dominant/submissive relationship, and her instability forces the central conflict out into the open. Without her breaking into Ana’s apartment or showing up at the masked ball, the external threat to their relationship would remain purely theoretical—it’d just be Christian and Ana arguing about contracts in a room.
Her actions catalyze everything. The stalking, the gunshot—these aren't subplots, they're the mechanics that expose Christian’s vulnerabilities. He can't control Leila like he tries to control everything else, and that loss of control is vital for his character development. It makes him confront his past treatment of submissives not as an abstract regret, but as a person who is now, actively, causing harm.
Honestly, I sometimes skim her scenes on re-reads because they stress me out, but that’s the point. She’s the consequence he never dealt with, and the plot can’t move forward toward any resolution until that consequence is physically, dangerously present.