In real life, if there was a masked vigilante whose codename is ‘Watcher,’ he’s a non-lethal masked vigilante who is a witness to a murder. The detectives and prosecutor want to put the vigilante on the witness stand, but the vigilante’s secret identity is, well, secret, and he doesn’t want to reveal it. Can he still testify with his mask on or testify anonymously?
In countries where the law is still observed, it is usually not possible. In order to have rights, you need to be someone. To have an identity. What an identity is, is defined by law. Pseudonym identities are very rarely legally possible. So your answer is No.
But if you are inside Usa (probably true because of your lack of saying it) then simply everything is possible, since they don’t have the rule of law anymore.