He gave a fearless smile.
Boldly, she went alone into enemy territory.
He gave a daring smile as he stared down his opponent.
She kept her defiant attitude even in front of her boss.
The criminal smiled defiantly even after being arrested.
勇敢 describes straightforward bravery and is usually positive, whereas 不敵 can carry a nuance of audacity or recklessness that 勇敢 lacks.
臆病 describes a timid, easily frightened disposition, the opposite of the fearless, defiant quality expressed by 不敵.
daring; fearless; audacious
Compound of 不 (not) and 敵 (enemy, opponent), literally 'having no enemy' or 'unmatched', which developed into the sense of being fearless or daring.