I have an Impact Character with a secret, and if it's found out, his life is over, just like a Columbo villain
In my story form his Critical Flaw is investigation.
I have it in mind to read this as he is vulnerable to being investigated and he can't avoid being found out eventually, all he can do is stave it off until such time as he is able to twist the truth in his favor.
Of course, he'll lose all credibility with the MC when his real nature is found out. The impact of finding out his lie will be awful but then his influence/impact over the MC will diminish to almost nothing.