Estelle, from Trails in the Sky.

She’s introduced as a bit of a cocky moron who constantly has to be wrangled by her more mature brother. But, there’s a slow development of competency where she starts to become decently sufficient at all the things her brother is good at, while he meanwhile lays bare some heavy emotional flaws - many of which Estelle excels at processing (call it a feminine trope, but it works).

Of note, all the rest of the Trails series have had male leads, and their pace of character development ground to a complete halt.