Spark
Sparks are an innate part of each and every character in TTG. Even those that do not actively make use of Favor in any way still possess one, only dormant. A character's spark is representative of their will and being, taking form as a unique ability to aid them when they put Favor to use. One's spark can often be a major clue towards their Favor#Alignment, as both are born of the same fundamental existence.
As a general rule sparks cannot be forcibly changed, although with the help of the divination system they can be replaced with constructed abilities. These abilities are much weaker, but a poor understanding of sparks made them commonplace during the heyday of the Divination System.
This does not mean sparks are static, in fact they are very flexible. A Spark will reliably adapt to any build, though it will still be fundamentally the same ability.