Attackers
The Left side of the combat window is reserved for the attackers or fight initiaters, typically all of the people + pets in your group are shown here, together with current Health and Mana so you can make an educated decision on the best individual to use that round.

If you have a pet, then attacking with your pet owner will cause the pet to attack automatically otherwise if you choose to attack with the pet it will attack on its own without the owner.

You can choose anyone in the left list to use whatever abilities, spells or styles they might posess for combat, the choice is up to you the entire fight rests on your shoulders !