What term is used to describe someone who collaborates with a hijacker without revealing their identity?

The term that best describes someone who collaborates with a hijacker without revealing their identity is "the accomplice." An accomplice is someone who assists another person in the commission of a crime, knowing that the action is unlawful, while attempting to remain undisclosed or anonymous. This involvement can include planning, providing resources, or facilitating the crime in some manner, all of which can occur without their identity being immediately detected.

In the context of hijacking, an accomplice's role can be crucial in executing the criminal plan, creating an environment that allows the hijacker to operate more effectively. This contrasts with other terms, which have different implications. For instance, a sleeper may refer to someone who remains inactive until a particular moment or situation arises, while a spy typically gathers intelligence for another party rather than aiding in a crime like hijacking. Similarly, an informer usually provides information about criminal activity to authorities. Understanding these distinctions clarifies why "the accomplice" is the most appropriate term in this scenario.