IMO, she's supposed to sound monotone (please note that I haven't watched the anime, because I try not to watch series if I'm working on the novel version so I don't get bored).
In text, the way they speak can be conveyed a number of ways, but with RyuZU:
We keep most of her dialogue uncontracted (for the most part, it's cannot instead of can't, etc). Part of this comes down to her background in general, but it helps give a feel of monotone/stuffiness, as well as vocabulary choice. When's the last time you heard someone with a huge vocabulary sound excited?
Her actions. In novels, you can just as easily say "she spoke in a monotone voice." The author hasn't done that yet, though, but lets her other actions speak for her. She often has a calm/blank look on her face and her movements are very calculated. Using past cues from other books people have read, it's easy to create the monotone feel based on other things aside from just how her voice would sound.
However, she does occasionally have (small) bouts of emotion. :)
