The Stalker is a common specimen in Killing Floor 2. It is classified as a lesser enemy, and therefore, cannon fodder.
Stalkers are unique in that they possess a cloaking device which renders them nigh-invisible to players. The killer you don't see coming, stalkers are able to bend light around them to make them nearly undetectable to the naked eye. They can be seen as a slight flicker when moving, only becoming wholly visible while attacking or when a Commando uses his perk ability to call the stalker out. Stalkers quickly close with their targets and attack before backing off, disappearing and approaching against from another angle.