Out of the options of Body Size and Composition, Muscle Strength, Muscular Endurance, Power, Speed / Quickness, Agility, Flexibility, Balance and Coordination, and Cardiovascular Endurance, the factors which are considered most important by the readers of this site are Balance and Coordination and Speed / Quickness.
The best answer to fill in the blank would be ''Bystander'' because as that role you can go tell any adult or higher authority to stop the fighting so you don't try to be a hero and get yourself hurt as well.