Use 'mettle' when discussing someone's courage or strength in difficult situations. She'll have the opportunity to test her mettle in the championship game.
Pro Tip 2/3
Positive Connotation
'Mettle' often implies a positive judgment of someone's capabilities. The firefighters proved their mettle during the rescue operation.
Pro Tip 3/3
Adversity Relevance
Mettle is most relevant when enduring hardships or pressure, not in ordinary moments. Surviving the crisis showed the whole team's true mettle.