: one who reveals something covert or who informs against another
especially: an employee who brings wrongdoing by an employer or by other employees to the attention of a government or law enforcement agency
Note: A whistleblower is commonly protected legally from retaliation.
I think whistleblowers play a hugely important role in countering the worst corporate and government excesses. Damon Poeter
… pledges to protect whistle-blowers who fear reprisals for their efforts. Wall Street Journal
Congress also added significant new protection for whistleblowers. The 1986 act provides remedies including reinstatement, back-pay with interest, and, as appropriate, punitive damages for whistleblowers who are discharged, demoted, or discriminated against due to involvement with a False Claims disclosure. Bradford A. Penney