Tara: Healing Charge
When a survivor on your team uses a charge attack that survivor will regain a percentage of their health. The percentage of health gained is dependent on the star level of Tara; the higher the level the greater the health gain.

Carl: Second Chance
Any survivor on the mission who would take damage which would reduce them below 0 health has a chance to avoid lethal damage rather than dying. The percentage chance of this ability is dependent on the star level of Carl; the higher the level the higher the chance of survival.
 
Aaron: Teamwork  
Successive attacks made by members of your team on the same target deal a certain percentage more damage. This percentage is dependant on the star level of Aaron; the higher the level the greater the damage dealt.  
 
Merle: Jackass
Your survivors deal a percentage more damage to enemies of any level higher than your team.  The percentage of increased damage is dependant on the star level of Merle; the higher the star level the greater the damage increase.