Can an employee request an observer for their employment review?

I am not sure if you wanted to have a witness attend the meeting of your review, or if you are asking about whether your manager can have someone attend your review. There is no rule that allows you to have someone attend your performance review.  If there are concerns you have, or you do not trust the manager, you can ask if the meeting could be recorded, but they would not have to agree to that. If you are worried that your manager had some clerk attend your review, that would not be proper.  It is not uncommon to have another supervisor or manager attend, or an HR person attend.  But, not a mere clerk who would be your co-worker