This video shows where to find which random traits your mercenaries have. Below is a list of all traits I have found in the game, up to this point.
Giant: Strength/Dexterity +5% when equipped with a 2H weapon
Lively: Strength/Dexterity +5% when equipped with a 1H weapon
Clever: +5% Experience
Hard-working: +5% Profession Experience
Night vision: See better in the dark
Tyrannical: Willpower increased by 2 as long as there are prisoners in the troop
Tough: Has a chance to not suffer injuries in combat
Bloodthirsty: +3% Critical Hit
Thick-skinned: +3% Guard
Opportunistic: +5% Damage to Attacks of Opportunity
Volunteer: Wages reduced by half
Solitary: +10% Damage if NOT next to an ally in combat
Cooperative: +10% IF next to an ally in combat
Strong: +5% to Strength
Nimble: +5% to Dexterity
Brawny: +5% to Constitution
Quick: +1 to Movement
Confident: +1 to Willpower
Duelist: Critical Hit and Critical Damage +5% when equipped with a 1H weapon
Tormentor: Critical Hit and Critical Damage +5% when equipped with a 2H weapon
Brave: +5% Strength and Dexterity if outnumbered at the start of battle
Glorious: Chance to generate 1 Valour Point at the of the their turn
Resilient: +5% resistance to Poison, Bleed, and Burning
Eagle-Eyed: +10% Precision (from sa sa)
Stoic: Not included in group's Happiness total (from sa sa)
Shield Connoiseur - +5 Guard when holding a shield (from sa sa)