so she can be trusted as far as we know. ok , ty for actually answering my question.
She does many things behind the protagonist's back and hides his plans, but so far, everything is still done in a way that favors the MC and strengthens his power within the empire.
Whether, like Alaina revealed herself, she does this solely to achieve a goal that benefits herself has not yet been revealed. However, so far, despite Cass imposing some things and keeping the MC in the dark at times, she hasn't done anything wrong against the MC or the Empire in general.
Cass makes many plans and executes them behind the scenes, often without the MC's full knowledge. Judging by the comments that followed, many dislike this and feel manipulated. But in any case, I think that's irrelevant to your question about Cass having power...