敢于直谏的(形容词)英文怎么说??

直谏，用admonish这个词比较贴切些。
敢于直谏的=someone who has courage to admonish the emperor to his face

另外，rebuke也有这样的意思，但程度上很重。例如：To rebuke one's superior to the face=犯颜直谏

没有这样的形容词
只有动词
have the courage to tell the emperor where he is wrong

outspoken 直言不讳

straightforward