Felicia is Mr. Alacrán's wife and she, more than anyone else, reveals what a horrible effect El Patrón can have on those around him. In fact, at times it seems like the only reason she even exists is to show just how awful El Patrón is. Can you think of characters in other books that might serve a similar purpose?
When we first meet Felicia, she's a pathetic and scary sight for sore eyes: "After a while Felicia would run out of energy. Trembling and pale, she would hunch over the keys, and this was the signal for a servant to bring her a brown liquid in a beautiful, cut-glass bottle." (9.12) You can't help but feel sorry for the woman. Poor Felicia is an alcoholic who appears very timid and afraid. She can barely get her words out much of the time. She's bossed around by the men in the family and seems to have no life of her own.
So it really comes as a shock when we learn that Felicia killed Furball in an effort to frame Matt. Turns out she's harboring a lot more rage and cruelty than we would have guessed, and it's all aimed at Matt, because he's El Patrón's favorite:
"I wanted to kill that abomination El Patrón keeps at his heels."
Matt felt cold. He'd had no idea how much Felicia hated him.
"But I had to be satisfied with [...] that filthy, slavering rat María called a dog." (16.88-90)
Felicia is so cold and ruthless, it's scary. Of course her maniac son Tom is impressed by her plan. And yet, despite all her hidden cruelty, we still feel sorry for her, because she's also a victim of El Patrón's obsession with power and control. Sure, she makes bad choices in a bad situation (an affair with MacGregor? Ick.), but she has so little control over her life that we sympathize with her, too.