Armor

Armor is a mostly obscure stat present on both Survivors and monsters. Having positive armor reduces the damage that character takes, and having negative armor increases the damage taken. This damage modification applies to all damage taken, except for the damage from a Shrine of Blood and damage from Survivors of the Void - DLC Content
Void Fog is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Void Fog.

Damage taken is multiplied by $1 - Armor / (100 + |Armor|)$ . This is a number between 0 and 2.

If a character has positive armor:

Damage taken is multiplied by $100 / (100 + Armor)$ . This is a number between 0 and 1.
The formula for the damage reduction is: $Armor / (100 + Armor)$ .

If a character has negative armor:

Damage taken is multiplied by $2 - 100 / (100 - Armor)$ . This is a number between 1 and 2. (Note that the negative armor is subtracted, and two negatives make a positive.)
The formula for the damage increase is: $100 / (100 - Armor) - 1$ .

The armor value that modifies the damage is a sum of the following contributions:

The value of the armor stat (base armor + modifications)
+adaptive armor
+300 if the character is an Empathy CoreEmpathy Cores
Recruit a pair of Solus Probes that gain power with more allies.
Every 30 seconds, summon two Solus Probes that gain +100% (+100% per stack) damage per ally on your team. and the incoming attack has the AoE damage flag. AoE is considered any blast attack, i.e., any attack that deals damage in a spherical volume. In addition, Unseen HandUnseen Hand (Secondary)Stunning. Launch a phantasmal hand from the ground dealing 600% damage. Drain, slow and damage enemies. Heals for every target hit.
Survivor: Seeker and the torus explosion of the Gilded Elites also have this flag, even though they are technically not blast attacks.

The modified damage is then decreased by 5 per stack of Repulsion Armor PlateRepulsion Armor Plate
Receive flat damage reduction from all attacks.
Reduce all incoming damage by 5 (+5 per stack). Cannot be reduced below 1.. Note that the final damage can never be less than 1.

Armor	Damage Reduction	Damage Multiplier
-80	-44.44%	1.44
-60	-37.50%	1.38
-20	-16.67%	1.17
-10	-9.09%	1.09
0	0.00%	1
1	1.00%	0.99
5	4.76%	0.95
10	9.09%	0.91
12	10.71%	0.89
15	13.04%	0.87
20	16.67%	0.83
30	23.08%	0.77
50	33.33%	0.67
70	41.18%	0.59
100	50.00%	0.5
200	66.67%	0.33
500	83.33%	0.17
1000	90.91%	0.09
5000	98.04%	0.02
10000	99.01%	0.01

It would take 330 Rose BucklersRose Buckler
Reduce incoming damage while sprinting.
Increase armor by 30 (+30 per stack) while sprinting. (+30 armor per stack while sprinting) to get a damage reduction of 99% while sprinting. This would amount to 9900 armor. Another way to achieve this amount of armor would be by obtaining 99 Survivors of the Void - DLC Content
Oddly-shaped Opal is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. Oddly-Shaped OpalsSurvivors of the Void - DLC Content
This content is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Oddly-shaped Opal
Reduce damage the first time you are hit.
Increase armor by 100 (+100 per stack) while out of danger. to get the same damage reduction while out of combat.

Armor Modifications

Most survivors and monsters have 0 base armor, with some exceptions:

AcridAcridAcrid is a melee-range hybrid who uses powerful poisons to melt tanky enemies.
Class: Melee / Ranged
HP: 160 (+48 per level)
Damage: 15 (+3 per level)
Armor: 20
Umbra: Test Subject, MercenaryMercenaryThe Mercenary is a high skill melee survivor that uses his many dodges to weave in and out of combat.
Class: Melee
HP: 110 (+33 per level)
Damage: 12 (+2.4 per level)
Armor: 20
Umbra: Hired Blade, LoaderLoaderThe Loader is a slow but powerful bruiser that can use her grappling hook to uniquely navigate the environment.
Class: Melee
HP: 160 (+48 per level)
Damage: 12 (+2.4 per level)
Armor: 20
Umbra: Bionic Powerhouse, REXREXREX is a half robot / half plant that uses HP to cast devastating skills from a distance. The plant nor the robot could survive this planet alone – but thankfully they have each other.
Class: Ranged
HP: 130 (+39 per level)
Damage: 12 (+2.4 per level)
Armor: 20
Umbra: Symbiotes, Seekers of the Storm - DLC Content
Seeker is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. SeekerSeekers of the Storm - DLC Content
This content is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.SeekerAs a meditative mid-range brawler, Seeker utilizes powerful healing to sustain herself and her team.
Class: Melee / Ranged
HP: 115 (+34 per level)
Damage: 12 (+2.4 per level)
Armor: 20
Umbra: Diviner of House Beyond, and Seekers of the Storm - DLC Content
False Son is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. False SonSeekers of the Storm - DLC Content
This content is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.False SonThe False Son is a slow but stalwart close-range warrior. With extreme health and defense, False Son can weather any storm. Use False Son's durability to stay in the fray and capitalize on his slow but deadly skills.
Class: Melee
HP: 180 (+54 per level)
Damage: 13 (+2 per level)
Armor: 20
Umbra: The Shattered Heir each have 20 base armor.
MUL-TMUL-TMUL-T is an aggressive survivor who has the tools necessary for any job!
Class: Melee / Ranged
HP: 200 (+60 per level)
Damage: 11 (+2.2 per level)
Armor: 12
Umbra: Right Tool for the Wrong Job has 12 base armor.
Boss Monsters have some base armor. See their individual pages for exact values.

The following calculations are then applied to this base value:

Irradiant PearlsIrradiant Pearl
Increase ALL of your stats.
Increases ALL stats by 10% (+10% per stack). multiplies this value by 1 + 0.1 ⋅ stacks.
Seekers of the Storm - DLC Content
Saving Grace is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. Saving GraceSeekers of the Storm - DLC Content
This content is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Saving Grace (Buff)
Increases stats by 7%. If health is low, heal 50% health and remove buff. multiplies this by 1.07.
Playing on Drizzle grants each player a hidden item which adds 70 armor.
Armor BoostArmor Boost (Buff)
Gain additional armor.Gain +200 armor. adds 200 armor.
Small Armor BoostSmall Armor Boost (Buff)
Gain additional armor.Gain +100 armor. adds 100 armor.
Survivors of the Void - DLC Content
Oddly-shaped Opal is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. Oddly-shaped OpalsSurvivors of the Void - DLC Content
This content is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Oddly-shaped Opal
Reduce damage the first time you are hit.
Increase armor by 100 (+100 per stack) while out of danger. adds 100 armor per stack while out of danger.
Jade Elephant45sJade Elephant
Gain massive armor for 5 seconds.
Gain 500 armor for 5 seconds. adds 500 armor while activeElephant Armor Boost (Buff)
Gain massive armor.Gain +500 armor, negating most attacks..
Survivors of the Void - DLC Content
Void Survivor Corruption is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. Void Survivor CorruptionSurvivors of the Void - DLC Content
This content is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Void Survivor Corruption (Buff)
Recipient has been corrupted.Indicator status effect used to show that the recipient is corrupted. This status effect is only given to the Void Fiend, whom has corrupted versions of its skills. adds 100 armor while Survivors of the Void - DLC Content
Void Fiend is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. Void FiendSurvivors of the Void - DLC Content
This content is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Void FiendThe Void Fiend is a corrupted survivor that fluctuates between a controlled and corrupted form, each with different strengths and weaknesses. Managing this curse has become its fate.
Class: Ranged
HP: 110 (+33 per level)
Damage: 12 (+2.4 per level)
Armor: 0
Umbra: Corrupted Amnesiac is corrupted.
Seekers of the Storm - DLC Content
Twisted Armor is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. Twisted ArmorSeekers of the Storm - DLC Content
This content is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Twisted Armor (Buff)
Hidden. Gain 300 armor.
(stackable) adds 300 armor to allies tethered to a Seekers of the Storm - DLC Content
Twisted is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. TwistedSeekers of the Storm - DLC Content
This content is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Twisted (Affix Buff)
Gain the power of a Twisted Elite.Grant +300 armor to nearby allies and charge a projectile when they are are damaged. elite.
CrippleCripple (Debuff)
Reduces armor by 20. Reduces movement speed by 50%. reduces armor by 20.
PulverizedPulverized (Debuff)
Reduce armor greatly.Reduces armor by 60. by Shattering JusticeShattering Justice
Reduce the armor of enemies after repeatedly striking them.
After hitting an enemy 5 times, reduce their armor by 60 for 8 (+8 per stack) seconds. reduces armor by 60.
Rose BucklerRose Buckler
Reduce incoming damage while sprinting.
Increase armor by 30 (+30 per stack) while sprinting. adds 30 armor per stack while sprinting.
Seekers of the Storm - DLC Content
False Son is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. False SonSeekers of the Storm - DLC Content
This content is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.False SonThe False Son is a slow but stalwart close-range warrior. With extreme health and defense, False Son can weather any storm. Use False Son's durability to stay in the fray and capitalize on his slow but deadly skills.
Class: Melee
HP: 180 (+54 per level)
Damage: 13 (+2 per level)
Armor: 20
Umbra: The Shattered Heir gains 2.5 armor per secondary skill charge available. This means that at the default 4 charges and with no other armor modifications, False Son has the highest starting armor of 30.
Survivors of the Void - DLC Content
Permanent Armor Reduction is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. Permanent Armor ReductionSurvivors of the Void - DLC Content
This content is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Permanent Armor Reduction (Debuff)
Permanently reduces armor.Reduces armor by 2 (2 per stack) for the remainder of the stage, or until killed.
(stackable) by Survivors of the Void - DLC Content
Symbiotic Scorpion is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. Symbiotic ScorpionSurvivors of the Void - DLC Content
This content is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Symbiotic Scorpion
Permanently reduce armor on hit.
100% chance on hit to reduce armor by 2 (+2 per stack) permanently. reduces armor by 2 per stack.
WeakWeak (Debuff)
Reduces armor by 30%, reduces movement speed and damage by 40%. reduces armor by 30.
Spinel Tonic60sSpinel Tonic
Gain a massive boost to ALL stats. Chance to gain an affliction that reduces ALL stats.
Drink the Tonic, gaining a boost for 20 seconds. Increases damage by +100%. Increases attack speed by +70%. Increases armor by +20. Increases maximum health by +50%. Increases passive health regeneration by +300%. Increases movespeed by +30%.

When the Tonic wears off, you have a 20% chance to gain a Tonic Affliction, reducing all of your stats by -5% (-5% per stack). grants 20 armor while activeTonic Buff (Buff)
Gain a massive boost to all stats.Gain the following boosts:
+150% max health
+150% max shield
+170% attack speed
+130% movement speed
+20 armor
+200% base damage
+400% health regeneration.
Seekers of the Storm - DLC Content
Growth Nectar is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. Growth NectarSeekers of the Storm - DLC Content
This content is part of the Seekers of the Storm DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Growth Nectar
Your buffs grant an increase to ALL of your stats.
Grants 4% increase to ALL stats for each buff, up to a maximum of 4 (+4 per stack). multiplies the above accumulated value by 1 + 0.04 ⋅ itemStacks ⋅ buffStacks

Adaptive Armor

MithrixMithrix
King of NothingHP: 1000 (+300 per level)
Damage: 16 (+3.2 per level)
Class: Melee / Ranged
Speed: 15 m/s
Armor: 20 and the Survivors of the Void - DLC Content
Voidling is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info. VoidlingSurvivors of the Void - DLC Content
This content is part of the Survivors of the Void DLC. It is only available if the DLC is enabled when starting a run.
Click for more info.Voidling
Diviner of the DeepHP: 2000 (+600 per level)
Damage: 15 (+3 per level)
Class: Ranged
Speed: 45 m/s
Armor: 20 have the adaptive armor passive stat, which protect them against bursts of damage. Every time they take damage, they gain temporary armor equal to $3000 \times (damage/maxHP)$ , or 30 armor per 1% of the max HP dealt. This armor caps at 400 and decays at a rate of 40 per second.