Aura (not to be confused with Light and Dark Auras, which are completely different things) is a slight energy force given off by everything alive. It is usually identified by color, but also has a scent unique to each person. This scent cannot be detected by most people, but a few people (Pika, for example) can smell it. People who can detect aura will keep record of the scent even if their memory is erased, causing people they've met before to smell familiar even though they don't quite remember who they are. They are also not easily fooled by people trying to pose as people they've met before, as they won't smell the same as the person they are impersonating.