party that challenges the defects or method.
1760, translating Latin advocatus diaboli, one whoever job it really is to encourage contrary to the canonization of a candidate for sainthood. "[F]ar from being the whitewasher associated with wicked, the [devil's recommend] may be the blackener for the good." [Fowler]
individual who identifies and challenges the defects in an evaluation, program, or strategy.
someone who takes the worse side only for the benefit of argument
If somebody plays Devil's advocate in an argument, they follow a posture they don't really have confidence in only for the benefit of the argument
They argued their way back to Parkside with Dean playing the devil's advocate while Fred quoted a dozen mystery stories that bore out his hypothesis, a hypothesis that grew in detail with each passing mile.