The term 'Observer of Yom Kippur' refers to a JEW, as indicated by the 3-letter solution. Yom Kippur is the holiest day of the year in Judaism, and is observed by fasting, prayer, and repentance. An Observer of Yom Kippur is someone who follows the traditions and customs associated with the holiday, such as abstaining from food and drink, attending synagogue services, and engaging in self-reflection.