Guardians (not to be confused with legal guardians who look after orphans) are characters that guard a treasure, rare item, or an entire area. They may be noble creatures but some are also malevolent. Some are totally amoral and have no personality outwith their role as a guardian. Once crossed, many guardians will not relent and may fight to the death to defend their prize.
Editing note: do not confuse Guardians with Protective Villains who protect and care about individuals. A Guardian is not necessary protective of anyone; in fact, most of them are rather Failure-Intolerant.