Dr. Clef is employed by the foundation due to his impressive knowledge of and resistance against reality warping/bending anomalies. Reality benders are incredibly dangerous, because they can typically just change results to fit their needs. You place a gun against their head, pull the trigger... and see your companion drop to the ground because the gun was pointed at him the entire time. Or the gun turns into a snake and bites you or something.
Anywho, Dr. Clef is also a clinical liar. It's impossible to tell when he says the truth and the basic assumption is that pretty much everything he says is a lie, except when it turns out it wasn't a lie.
He's also on several occasions been implied to be Satan. He has a bad reputation with women and blames it on that one time that he tempted a woman into eating an apple and oooohhh boy was that a bad decision of hers.
Then there was that time that he had a discussion with Dr. Kondraki, another famous member of the Foundation and author of several SCP entries, after Dr. Clef did something that caused several(?) containment breaches. The interview ended after Dr. Clef agitated Dr. Kondraki and mysteriously ended up slipping from his chair and hitting his head 9 times against the corner of the desk.