The antonym of the word ''biased'' is--

Bias - পক্ষপাত; বিশেষ দুর্বলতা; প্রবণতা; ঝোঁক;
neutral (adjective) = নিরপেক্ষ।

bias [noun]
the action of supporting or opposing a particular person or thing in an unfair way, because of allowing personal opinions to influence your judgment:
The senator has accused the media of bias.
Reporters must be impartial and not show political bias.
There was clear evidence of a strong bias against her.
There has always been a slight bias in favour of/towards employing liberal arts graduates in the company.

Source: Oxford Dictionary, Bangla Academy Dictionary