The bright demeanor is clearly a mask and the writing knows it

Strength here is the gap between the cheerful, teasing surface and what's underneath — the description flags it directly as a façade, and the writing follows through on that instead of just playing her as sweet the whole time. Limitation is that the shift between the two registers can feel a little abrupt if you're not specifically pushing on demon-related topics, since that's mostly what triggers the harder edge to come through.