Formal:
* Valiant: implies great courage and determination in the face of danger.
* Intrepid: suggests fearless and bold, especially in dangerous situations.
* Audacious: implies a willingness to take risks and act boldly.
* Dauntless: means fearless and not easily discouraged.
* Gallant: typically used for someone who is brave and chivalrous.
Informal:
* Bold: suggests confidence and willingness to take risks.
* Fearless: simply means not afraid.
* Gutsy: implies bravery in the face of danger or difficulty.
* Spunky: suggests a lively and determined spirit.
* Gritty: implies determination and resilience.
The best synonym to use will depend on the specific context and the nuance you want to convey.