intruder; infiltrator
Refers to a person who secretly enters a place, organization, or system, often with a specific purpose such as spying, sabotage, or unauthorized access.
The security guard found an intruder in the building.
He was sent into the enemy organization as an infiltrator.
Compound of 潜入 (せんにゅう, 'infiltration') + 者 (しゃ, 'person'). The word is transparently derived from its components.