General antonyms:
* Timid
* Shy
* Hesitant
* Diffident
* Bashful
* Modest
* Humble
* Submissive
Antonyms related to boldness:
* Cowardly
* Fearful
* Apprehensive
* Discreet
* Cautious
* Reserved
The best antonym will depend on the specific context in which you are using the word "brazen."