infiltrator

a soldier, intelligence agent, or other person who moves surreptitiously and gradually into an organization, territory, community, or the like, especially with hostile intent:Using a false name, the infiltrator participated in meetings and prep sessions with the protest group and had complete access to their documentation.

a person or thing that causes one substance to pass into another by filtering:The roots of wetland plants serve as massive water infiltrators, allowing more rainwater to seep into the soil and recharge underground aquifers.