Cassidy knows it all too well at this point.
Since Cecile has always been framing others for her crimes and putting the blames on them, she's already expecting that she's going to do the same thing right now. There's no use denying the truth that she's been locked inside her room and everybody in the mansion was in it.
It's also a good thing that Adrian believes Cassidy right away. Since it's only recently that he begins treating her amiably, she's still not very confident that he's going to side with her. Then again, she's still not even used to the way he's acting all generously by lavishing things on her.
"Cecile..."
As soon as Adrian says her name, Cecile straightens her back, tensing up even more. His tone is tyrannical and so is his expression. She used to see him like this a number of times before, but it's never directed to her or any of them.