WARNING *****************check bottom of solution for advice regarding the final boss as your cold value is releated to 3 achievements which are only obtainable in the final boss battle. You are supposed to playthrough the game twice to earn these however there is a way to get all 3 in one game. scroll to end for advice on this******************
Coldhearted achievement
These are the range of Cold values, you can't tell your exact value but if you go onto the attribute screen it will tell you your current range. Cold level goes up to 100, you need 90 for the achievement.There is a bug in the game when your cold value is between 80,-90 your status will say emotional, dont worry this is a bug it should say Ice cold, others have reported that it can also say this at cold value 70-80.
The best way to increase your cold level is through dialogue. Save often if succesful it will say Cold Level Increased, if it says decreased reload and choose an alternate option. think of Cold as the same as the emotion, so in dialogue try to be as unemotional and 'cold' as possible, be selfish, obedient, unapologetic and uncaring. eg, if someone says they want someone dead, say, 'how much do I get', if you are asked to leave an area, dont protest say OK, and so on. If you have been following the story you will know Jax once belonged to the Albs, they are fed Elex which deprives them of all emotion leaving them focused solely on the mission objective. It takes a while to get used to as the computers first choice of dialogue is emotional so always choose the dialogue, after a while it will become obvious.
You can also boost this by drinking Elex potions, you should be crafting these as much as possible, I recommend finding the Origin base and buying yourself a crafting table and merchent, the merchent has an inexhaustible supply of natural elex, use this to craft Elex potions which not only increases your Cold level slightly but these potions give you extra attribute points or skill points. I also recommend getting to Animal Trophy 3 quickly, this gives you huge amounts of materials every kill which you can sell for more Natural Elex. Purchase Sunglasses ASAP, they reveal loot even through walls, these will spot you weapons and resources you would have missed and are essential.
There is a bug in the game, sometimes your cold level will say Purely Emotional, dont panic this is a bug, you will either be Logical, or mellow, go off the progress bar.
0 - purely emotional
1-10 - expressive
11-20 - irrational
21-30 - spontaneous
31-40 - intuitive
41-59 - neutral
60-68 - dispassionate,
70-78 - mellow,
80-88 - logical, warning the 89th text is bugged and states purely emotional
90-99 - ice cold
100+ - synthetic
WARNING Make sure you get the achievement for cold value of 10 before this. it is much easier to increase cold value than decrease it. There are 3 achievements related to the final mission. if you go into the final boss battle with a cold value that is too high, you wont be able to get all 3, I missed out on the achievements the Third Way and Pure Rebel. To get all 3 in one playthrough, you must reach the final boss with a low cold value, you will need 100 ELEX potions per 10 cold value, so if you go into final boss battle with 50 cold value you need 500 potions to get Cold to 100.. the first achievement