Planescape: Fallout/Experience

To advance skills:

• It costs 3 stars to increase one skill by one point, but:
• Any time you increase a skill, your new skill pyramid must have at least as many skills at any level as the level above it.

This means you can't immediately bump a +3 to a +5, because to do that you'd need to increase it to a +4 first, leaving you with two skills at +4 and only one at +3.

So, if you want to increase skills, you must start by increasing a +0 to a +1, and then after that you can increase another +0 to +1 or a +1 to +2.

Alternatively, you can buy stunts!

• It costs 5 stars and an in-game reason to buy a new stunt.

So, Hark probably can't buy 'flight' without finding a jet-pack or something, but Zahra could theoretically buy a new vampire ability she recently grew into as a result of some in-game excuse like exposure to a new type of sunlight on a new plane or something.