At times, Mariana seems as if she's simply been corrupted by César and is only following his lead. However, the flashbacks tell a different story, as we see Mariana learn that Sara is carrying César's child. The pair then go on a walk where Mariana contemplates pushing Sara off a cliff before ultimately conspiring with Elroy to get rid of the girl. It is Mariana who suggests Sara try parasailing and Mariana who hands the knife to Elroy in the hopes he'll tamper with the harness.
Through flashbacks, we learn Elroy doesn't use the knife, but Mariana assumes he did. She then cleans up the scene of the crime while pretending to call an ambulance for Sara. In the present, Elroy confesses to Mariana that he did not kill Sara and that he wasn't able to go through with their plan. From her reaction, it's unclear if she thinks he's lying or is afraid he's telling the truth. After his confession, she murders him in his hospital bed, effectively silencing him forever. Despite having strong motives for being the killer, and a plan in place for the murder, Mariana did not kill Sara.
