There are a number of personality trait that would be an asset to a forensic psychologist.
Firstly, impartiality is important as one should never jump to conclusions without first considering all the evidence.
Secondly, the ability to distance yourself emotionally from a situation as emotional attachment can often cloud your judgement and make it difficult for you to think objectively about a case.
Finally you will need a sharp mind and the ability to adapt your ideas based upon what you learn throughout a case in order to ascertain an accurate and unbiased view of the situation.