Here is the core of character progression — Classes. Each character has a Class that grants 7 usable abilities, as well as a number of stat boosts. When you earn XP, you can only spend it on abilities and stats related to that class.
However! You can change class pretty much anytime. When you first start out, you have access to only 3 classes:
The first 3 are Warrior, Scout, and Scholar. Once you earn 200 XP as a Warrior, though —
Your 200 Warrior XP must be spent unlocking or improving Warrior abilities, or on stats related to the Warrior (such as Physical Attack, Physical Defense, Sword Skill, etc). Once you change into a Brawler you can begin learning Brawler abilities.
In addition to choosing a class, you can also equip a 2nd class’ skillset. You don’t gain any stat bonuses from the 2nd class, though, just the usable abilities. If you are a Brawler/Warrior, for example, you can use abilities from both classes. With 17 different classes to choose from, you can deeply customize each of your characters. Enemy humanoids have the same opportunity though, so watch out!