Mercenaries statistics are very important for what they are going to be successful at have a guy with three talents or ‘stars’ for Melee Skill then on levelup he could expect to gain 3-4 additional Melee Skill if you choose to invest in it.
So let’s breakdown the various stats and how useful each one is,
Hitpoints when these hit 0 you’re dead so the more you have the more hits you can take also the larger the pool the less likely to suffer injuries from being hit. Characters can sometimes survive battles where their total hitpoints were reduced to 0 or below but will suffer a permanent injury, such as Brain Damage.
Action Points this isn’t so much a stat that can be upgraded just know that all characters have 9 at max and some can have less if they have been injured/poisoned. You need these to move, attack, and use abilities/skills.
Fatigue is another resource like stat that is used to move, attack, and also for skills/abilities you recover 15 fatigue each turn regardless of what you did that turn, unless some other effect takes place or helps recover more. (Iron Lungs, Recover, some injuries)
Morale from Confident Morale which will give you buffs to combat effectiveness to fleeing there are 5 states of morale that can be changed as the battle progresses depending on morale checks triggered from : killing an enemy, seeing an enemy be killed, seeing an ally be killed, seeing an ally flee, being hit with 15 or more damage to hitpoints, being engaged by more than one opponent, use of skills like ‘Rally’ all of these checks affect morale for both side potentially only some enemies feel the effects of morale but your men always will.
Resolve Higher resolve more likely to be of confident morale, lower resolve more likely to flee from negative events/outcomes, this stat also acts as a defense against mind control, fear, panic.
Initiative the higher this stat the earlier your turn will be available in the round it’s affected by fatigue, and weight, so for example an exhausted, heavily armored merc will be going last in the round even if he normally has a higher initiative.
Melee Skill this is your chance to hit an enemy with a melee attack
Ranged Skill your chance to hit with a ranged weapon attack
Melee Defense the chance to completely avoid an enemy attack
Ranged Defense chance to completely avoid an enemy ranged attack
Vision the amount of tiles you can see affected by traits, helmets, injuries.
So once you know what type of talents a character has and if he has any unique traits you can start to create a specific role for him to fill within your company, that mercenary who has talents in HP, and Melee Defense would make a great frontliner and you should prioritize his relevant stats according to his role i.e. Melee Defense, Hitpoints, Melee Skill, and Fatigue being the most important. My personal preference is after a decent amount of hitpoints has been accumulated I focus on maximizing fatigue and the relevant attack skill and Melee Defense maybe even putting a couple into Resolve because morale has undergone some important changes and is now more relevant.
My overall opinion on which Stats are most important are as follows,
Most Important Stats Fatigue, Melee Skill/Ranged Skill Depending, Melee Defense and Hitpoints
Resolve is more important than it previously was but it shouldn’t take precedence over any of the more important stats except for on Morale Buffers like Sergeants or guys who really need it for some reason. Which leaves Ranged Defense and Initiative, Ranged Defense is naturally less useful than Melee Defense because the sheer majority of enemies encountered are Melee Oriented and being better suited to dodge those attacks simply makes more sense. Initiative on the other hand is the least important statistic of all especially when considering perks like Adrenaline allow you to act before your enemy when you want with only the cost of additional Fatigue, which funny enough only makes Fatigue even more important and Initiative even less.