Generally, higher ranking classes have better stats and skills, but this is not always the case. Higher classes also tend to be harder to obtain from the Tavern.
There are 7 classes, with Z being the highest and D being the lowest.
How to obtain
Adventurers are Z, SS, S, A, and B classes. These characters tend to have access to up to three jobs. They can be obtained through the Pact of Truth, story progress or in-game events. A few Adventurers can also be obtained from the Pact of Fellowship.
Some characters and companions are promotional and cannot be obtained by regular means in-game. They can only be obtained through gift codes from external purchases or events.