steely
adj.having a very strong and calm character, especially when facing a difficult situation. It describes someone who is brave and does not show fear.
adj.possessing a firm, resolute, and unyielding character. Often used to describe a person's gaze, voice, or determination in the face of adversity.
“She gave him a steely look that made him stop talking.”
“Despite the pressure of the final exam, she maintained a steely focus on her notes.”
“The general's steely resolve during the siege prevented the troops from retreating, even when supplies were running low and the enemy was closing in.”
“And behind her image, she could be a model of smart, steely self-determination.”
From steel + -y.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.