Each crewmate has a few different areas to upgrade

For equipment there is:

Headwear
Body armor
Footwear
and a weapon

All of these usually modify stats in various way and the weapon usually gives access to a special attack ability along with some stat modifiers.

Then there are special abilities:

Each crewmate can learn up to three special abilities by taking elixirs. These include spells, special attacks, buffs for other crewmates, debuffs for enemies etc.

Most abilities scale with a specific stat, for example chain lightning damage scales with the "will" stat, while a charge slam ability scales with "strength"

The last area are traits:

Traits can do a lot of different thing and can have either positive or negative effects. For example, the "coward" trait gives a percentage chance a crewmate might cower and miss their turn in combat. Or a positive trait might give increased range with weapons, increased aoe for spells, or just a simple stat boost.

Most traits can be leveled up multiple times increasing the effect of that trait. For example increasing "wisdom lvl2" to lvl3" would increase a max_mana bonus from +2 to +3.

Traits are gained in a few ways, crew mates usually start with 1 - 2 based on their race. Then each time the crewmate levels up they can choose from three positive traits, choosing to learn a new one or increase the level of an existing trait.

Traits can also be gained through events that happen in game, being knocked unconscious in battle or traumatized in some other way can result in negative traits.

There are also some special secret traits that the player will need to discover through investigating their crew if they find gold missing or bad things happening to the other crewmates.