Knights are a job that originated during the war by dream types who were the first to discover the act of boosting. At that time there was no name for this job and it was less refined as the top priority of everyone was to survive the war. When the war ended under the order of Isadora these abilities were refined and developed into a job known as Knights. 

Knights come in 5 classes each with its own unique purpose and skill. Fluffs who are knights are respected in Asterolia and are considered the protectors of the region. If your character is a Knight and has any of the classes it means canonically they graduated from the Knight Academy which must be incorporated in their backstory. Knights can take missions and requests from the Knight Headquarters where a supervisor can inform them of new missions they must/can take up. 

Starter characters cannot be knights but can wish to study to become one or are studying currently. 

Mercenary are what Nightmare types in Darkalia call their own knights. Originally they were named anti-knights, a joke name for the fact they stole all the classes from the Dream types and made them the opposite of what they were originally. However, as time went on, they eventually became legit classes and unique to Nightmerfluffs, now part of Darkalia's culture. Mercenary do not have headquarters or an academy and is instead a group of people that join a guild and help others, usually fluffs seek those knowledgeable in these classes to teach them the class. Their emblems are unofficial and usually groups will have them as tattoos if they would like to. 

Mercenary come in 5 classes. Fluffs who are Mercenary gain respect and reputation via helping everyday fluffs and fighting for them instead of gaining a title via graduation (which is what they assume Dreams do). Might come as a surprise that they go around helping considering their classes sound more savage. They are mostly found in Darkalia as that is where they originated from, but since they are a guild, many of them can be found in other places. Mercenary can take missions and requests from their guild and bulletin boards.

Starter characters can be mercenaries. 

Mages are fluffs that study under the Academy of Magic and usually swear to dedicate their lives to magic. Unlike Knights or other jobs they exclusive use magic for things and find ways to incorporate magic in their daily lives. They are one step above an everyday fluff using simple magic and a Knight who uses boosting. They are essentially the person you go to about magic matters. Mages and Knights are in good terms as they cooperate in a lot of things including missions.

Mages that are independent , as in, they do not follow the academy, are usually fluffs that picked up magic as a hobby or learned through other methods. Academy mages tend to avoid them as their ideologies can vary, one of them being a type called "Witches".

Witches are fluffs that practice magic without overstepping its boundaries. As in, they will not go beyond what's needed which is what Mages are known for. They believe magic comes with a price and must be respected. They are closer to everyday fluffs as they tend to be helpers in different communities and appreciate nature. Witches are a way of living and is a job many outside of traditional cultures do not take seriously, usually dismissing them as a "relaxed mage" version. 

Witches were indeed fluffs that started off as mages. However they branched off after seeing the differences in ideology. The first witches left the academy and became independent, slowly forming their own identity and eventually forming the name "Witches". 

Witches unlike Mages do not share their secrets. Their way of living and methods will only be taught to those in their covens or apprentices. Becoming an apprentice of a Witch thought is much harder than becoming an apprentice of a Mage, so most can go years before managing to get a witch to teach them. 

Witches not available to make currently