Rosa seems to be in the running for the ultimate villain at the beginning of the book. When we first meet her, she's a housekeeper at the Alacrán estate, and she shows sympathy to a young, injured Matt. But when she learns that Matt is a clone, she changes completely.
It quickly becomes clear that Rosa is one cruel lady. She jails and tortures Matt for months in poor conditions, which traumatizes Matt and even gives him asthma. Evil and selfish, she blames Matt for "ruining" her secret romance with the doctor, Willum, which only adds to her meanness. We think Rosa definitely deserves to be punished for what she did to Matt. And she is. But is it a fair price to pay?
When Matt visits the stables in Chapter 15, he's horrified to find that, because of what she did to him, Rosa has been made an eejit: "The hard, bitter lines of her face seemed unconnected with anything going on inside. Rosa gazed straight ahead with her hand outstretched. It was unclear whether she even saw him" (15.6).
Despite how poorly she treated him, even Matt understands that no one deserves to be turned into an eejit, no matter how terrible her crimes. Rosa may have been cruel, but she was still a human being, and after she's turned into an eejit, she's not much more than a robot.