True, dumb mutes don’t really make interesting characters on a page to page basis. Ever since the Eclipse though, Miura has obviously been using her as a plot device to emphasize Gut’s struggle to choose between protecting what he loves or being consumed by hatred and revenge. She isn’t much of a character in her own right, but she serves as a powerful point of reflection for Gut’s motives. In that way at least, she’s pretty essential to the story right now.
Having said that, if it does turn out that she hates Guts, then he won’t have a reason to protect her anymore, and his hatred will probably win out in his struggle. Then you’ll definitely get all the rage you can ask for. However, when you put it that way, I really doubt that’s what Miura is planning on doing.