stalker
noun /ˈstɔːkə(r)/
  /ˈstɔːkər/
 - a person who follows and watches another person over a long period of time in a way that is annoying or frightening跟踪者;盯梢者 
- a person who follows an animal quietly and slowly, especially in order to kill or capture it悄悄接近猎物的猎人