Psychotic makes excellent points about Cid's character, all of which I agree with. I also think it's important to reiterate that Shera is his caretaker. She cares for him because she wants to and chooses to do so. As mentioned earlier, she isn't there against her will, she could leave at any time.

Painting Shera as a victim who isn't able to stand up for herself or know what's right for herself doesn't give her credit as a character, nor does it give her credit as a person. Think about the nature of caretaking people for a moment. Caretakers do not put themselves first. They do not do this because they are unknowing, weak people who do not understand the situations they put themselves in. They do this because they have a need to care for others more than themselves. To anyone outside a relationship between two people, it might manifest as abuse. That is because we project our own experiences onto their relationship in order to translate it into what we want to see it as, or to translate it as the experience we went through. It is clear that Shera is acutely aware of Cid's flaws as well as the fact that he takes them out on her, and that she chooses to overlook that. She likely understands that even though she herself made a mistake, Cid is the cause of his own happiness. It's a part of his nature as a person, and she understands that she will need to accept this if she is to remain a part of his life to care for him, which is what she chooses to do.

Responding to a person in the manner that she responds to Cid does not make them weak, it makes them strong. She knows that he needs her, he knows that he needs her. Does she deserve better? Yeah, probably. But she is there because she wants to be.