From Ex illis wiki
Aggressiveness (AGG) represents the warrior’s willingness to jump headlong in the thick of the melee. Units with high Aggressiveness are more likely to be the target of enemy attacks. Also, ranged attackers shooting at close range (inside the same melee) can accidentally hit their allies if they have a higher Aggressiveness than theirs. Finally, highly aggressive units are the last ones to be denied their Cut Down and the first ones to be denied their Charge Reaction due to the presence of enemy units in the same zone.
AGG is shown by the unit’s position on the screen: the higher the Aggressiveness, the closer to the enemy it appears.
This value does not increase with Levelling.