Introduction and General Information:
This guide is designed to primarily serve as a center for information on all things related to the hero mercenaries in Fortress Survival Alpha. They are trained from the Command Center building, under the second tab.
The six current heroes in Fortress Survival are Marine, Rocketeer, Tinker, Shadow Sniper, Alchemist, Commander, and soon to be Astronomer.
When you load your code, you may have some items on your survivor, such as Wings of Behemoth, Slujj Hat, Elemental Stone, Trojan's Roar, or what have you. The items I've found that help a hero out in some way are: Slujj Hat, Silvering, Wings of Behemoth, Elemental Stone, both Benign and Malignant forms of Bloodfire Virus [use on hero], and I suppose Horn of Voortr'hal if you're lucky enough to either have one in your code, or someone with one drops/gives it to you.
The most notable among these items is the Elemental Stone, which gives (minutes in game/20) +1 stats. If you keep going further into Endless Waves, it'll keep getting stronger.
The vendor building, also under the second tab, has a few items that benefit heroes as well. Under the basic/simple/first list, the single tome* and Staff of Teleportation, which lets you teleport the unit holding the staff to an allied structure [not sure about unit], are the helpful items, but the second list holds far more helpful items.
*Tomes increase the hero's primary stat by 15 points and their secondary stats by 5.
Under the advanced/second list, the 10x tome bundle, Black Larceny (increases damage by 300 and attack speed by 300%), Rainbow Blasting Staff (deals 2260 magic damage per second to nearby enemies *note this damage is split between enemies*) [can also be automatically combined with Elemental Stone to create the Elemental Blasting Staff, which has the same stat gain ability of Elemental Stone and deals 6780 magic damage per second to nearby enemies], and Health Stone (increases HP by 6000 and HP regen by 200 per second).
All heroes have blink or some variation of it.
Abilities where I say something like "Mana-cost: 110 +30/marine; max of 290" means that it has an initial cost of 110 mana.
Do note: Intelligence does actually affect mana regen, but it's not very much.
Agility doesn't increase attack speed.
Onto the actual heroes!
Marine: 
Basic, well-rounded hero with many unique abilities (Rank 1). Agility-based.
Basic stats:
80-98 Pierce damage, 650 range. Fast attack speed.
Strength: 39, +6/lv.
Agility: 36, +9/lv.
Intelligence: 27, +5/lv.
Abilities:
Summon: Marine Squad (20 levels)
Summons 5 marines, with +1 marine every 3 levels of the ability (maximum of 11 marines). They receive upgraded health, armor, and damage every 5 levels of the ability.
Duration: 35 seconds.
Cool-down: 45 seconds.
Mana-cost: 110 +30/marine; max of 290.
Summoned Marines start with 2000 HP, 261-266 Pierce damage, fast attack speed, and 700 range. They also have a 3% chance for a 1 second stun. Every time they receive the upgrade built into the ability, the marines gain HP, armor, and damage. The max at 7500 HP, armor is negligible, and damage at 290-295.
Notes: Helps clear enemies at walls quickly.
Headshot (20 levels)
Damage: 1200 +450/lv; max of 9750. 4 second stun.
Cool-down 20 seconds.
Mana-cost: 125.
Notes: Effective against early bosses and some later ones if you need a few seconds to repair your walls *cough Lt. Stealer cough*
Quadruple Grenades (20 levels)
Damage: 80x4 AOE +30x4/lv; max of 650x4.
Cool-down: 3 seconds.
Mana-cost: 42 +2/level; max of 80.
Notes: I've never found this ability to be that helpful. Those that would generally be using Marine hero would have a lower rank, therefore lower mana regen. With low mana regen, it's quite hard to effectively spam Quadruple Grenades.
Laser Strike
Damage: 1000 +100x agility; max of 200k damage.
Cool-down: 240 seconds.
Doesn't use mana.
Ion Cannon
With a trip to the Librarian in the Library base you can purchase a tome to upgrade the Marine's Laser Strike ability. This tome costs 2 gold and 100 lumber.
Damage: 10k AOE + 125x agility to target; max of 300k damage. 3 second stun to target.
Cool-down: 240 seconds.
Doesn't use mana.
Notes: Nice ability to either severely damage or kill a boss of your choice. Takes out a good chunk of Lt. Stealer's HP, especially if you have Ion Cannon.
End notes: Balanced attribute growth. Marine seems to do better against bosses than against normal enemies. I personally prefer Rocketeer to Marine.
Rocketeer: 
Fully offensive hero with splash attacks (Rank 1). Strength-based.
Basic Stats:
119-147 Siege damage, 650 range. Fast attack speed.
Strength: 42, +11/lv.
Agility: 28, +3/lv.
Intelligence: 26, +4/lv.
Abilities:
Adrenaline (20 levels)
Increases hero's attack speed to the maximum amount for 9 + 2x ability level seconds. Maxes at 49 seconds of up-time.
Cool-down: 90 seconds.
Doesn't use mana.
Notes: Helpful if you need to clear out a large number of enemies, but don't have a Black Larceny to increase attack speed.
Launch Nuclear Missile (20 levels)
Damage: 750 +350/lv AOE; max of 7400. Slows down enemy AS and MS drastically.
Cool-down: 120 seconds.
Mana-cost: 80 +10/lv; max of 270.
Notes: Because of its global range, you can use this ability to help out an ally in need, without having to move your hero over there. Takes 4 seconds to connect.
Rocketstorm (15 levels)
Damage: 35 +10/lv per rocket [total of 90 rockets]; max of 175 per rocket.
Duration: 5 seconds.
Cool-down: 60 seconds.
Mana-cost: 60 +10/lv; max of 200.
Notes: Aim close to hero for wide spread, aim far away from hero for a compact area-of-effect.
Throw Grenade (20 levels)
Damage: 300 +150/lv AOE; max of 3150.
Cool-down: 2 seconds.
Mana-cost: 31 +1/lv; max of 50.
Notes: Similar to Marine's Quadruple Grenades. I find this ability even less helpful for the same reasons and the fact the Rocketeer has splash damage to begin with.
End notes: Focus on strength attribute, giving high health and damage. Abilities are made to be used on normal enemies rather than bosses. Rocketstorm and Throw Grenade aren't that usable towards the end-game phases, where either Rocketeer is tomed-up, or you have a lot of towers to put out damage, or both.
Tinker: 
Excellent base-support hero (Rank 4). Intelligence-based.
Basic stats:
135-147 Magic damage, 600 range. Fast attack speed.
Strength: 18, +3/lv.
Agility: 21, +4/lv.
Intelligence: 46, +11/lv.
Abilities:
Cogs of Time (10 levels)
Makes survivor instantly build next structure in his build queue.
Cool-down: 180 seconds -10/level; max of 90 seconds. (Going to be 50 seconds -3.333/level; max of 20 seconds.)
Mana-cost: 25.
Notes: Very helpful to start up a lumber setup, as the first one built takes the longest. In my opinion, it begins to lose value as the game progress, perhaps still usable for a later vendor or towers.
Reconstruct (20 levels)
Repairs structures or the Robo-Tinker at a fast rate.
Heals: 40 x ability lv + .25% max HP of target; max of 800 + .25%.
Cool-down: .2 seconds.
Mana-cost: 10.
Notes: Useful to panic-repair a wall, or the Robo-Tinker if need be. Mana shouldn't be too much of a problem, as Tinker hardly uses mana save Reconstruct.
Robo-Tinker (20 levels)
Transforms the Tinker into a Robo-Tinker, giving him Chaos damage, increases damage ever so slightly, but reduces Tinker's range to melee. Also gives Tinker a 10% thorns aura.
Strength: +100/lv; max of +2000 strength.
Armor: 31 +1/lv; max of +50 armor.
Notes: useful for early tanking and the only way Tinker can deal damage to the magic immune stages of Trojan/Najort/Behemoth. In my tests, it was do-able, but did take some time to kill them.
End notes: Focus on intelligence growth. From what I've seen, while Tinker isn't my favorite hero, the criticisms that he's completely or nearly useless are unfounded. He was able to deal out decent damage throughout the game. Things did get close with Najort, but I still managed to kill it, and Behemoth later on.
Shadow Sniper: 
Advanced sniping hero with powerful single-target abilities (Rank 7). Agility-based.
Basic stats:
146-147 Pierce damage, 900 range. Average attack speed.
Strength: 28, +4/lv.
Agility: 57, +13/lv.
Intelligence: 16, +3/lv.
Abilities:
Shadow Cannon (20 levels)
Damage: 800 +400/lv; max of 8400 to one target in area. Linear.
Cool-down: 8 seconds.
Mana-cost: 46 + 6/lv; max of 160.
Notes: Effective enemy clearing ability. Loses use as the game progress, when towers or the Sniper himself is dealing damage more consistently. Apparently has a cap on damage at 99999 across the whole area (I'd like clarification on this).
Adrenaline (20 levels)
Exactly the same as Rocketeer's Adrenaline ability. End of story.
Notes: Helps Sniper more than Rocketeer, as Sniper has a slower attack speed.
Critical Shot (20 levels)
+1%/lv chance for 5x critical hit.
Notes: Great passive ability. With luck, the end-game bosses will die very quickly if you get several criticals in a small proximity to each other. Never loses use in game [although it's not too helpful in the beginning stages].
Umbra Modification (20 levels)
Transforms the Shadow Sniper into a Umbra Sniper, which increases damage slightly, gives hero Chaos damage, heals the Sniper, and gives the Sniper area-of-effect damage.
Duration: 16 seconds +1 second/lv; max of 35 seconds.
Cool-down: 275 seconds - 5 second/lv; maxing at 180 seconds.
Mana-cost: 200 mana.
Notes: Effective against bosses, large numbers of enemies, or high-HP enemies *cough Endless Waves cough*. Also helpful, but make sure this isn't the only reason, as a heal.
End notes: Strong focus on the agility attribute, giving high damage and armor. Effective against enemies at any stage of the game. Worth noting that Shadow Sniper has a modified Blink: Dark Warp, which has a range of 2500 and a 10 second cool-down.
Alchemist: 
Excellent support hero with alchemy (Rank 11). Intelligence-based.
Basic stats:
139-151 Hero damage, 625 range. Fast attack speed.
Strength: 27, +3/lv.
Agility: 24, +4/lv.
Intelligence: 40, +14/lv.
Abilities:
Overgrowth (10 levels)
Deals area-of-effect-over-time damage.
Duration: 3 + .5 second/lv; max of 7.5 seconds.
Damage: 250/second +15/second; max of 385/second.
Cool-down: 120 seconds.
Mana-cost: 200.
Notes: Decent crowd-control and damage to earlier enemies. Area increases with each level.
Reinforce (unlocked from start)
Heal: 40% of structure's max HP. Structure is invulnerable for 2 seconds.
Cool-down: 600 seconds. [Perhaps 300 seconds, because of the the cool-down timer's appearance.]
Doesn't use mana.
Notes: Despite the long cool-down, it's a decent panic heal to a wall.
Transmute (unlocked from start)
Makes a target mill into a tiny version of it, which takes less room.
Cool-down: 1 second.
Mana-cost: 45.
Notes: I've only found it worthwhile to use for singling out one mill to be used for a gold mine. This must be used for all tiny versions of mills if you wish to move them; also for normal sized level 5, 6, and 7 mills.
Mass Transmute (unlocked from start)
Makes target mills in an area into tiny versions of themselves.
Cool-down: 7 seconds.
Mana-cost: 90.
Notes: In nearly all cases, more effective [and less annoying to use] than Transmute. Area effect can be manipulated to give it much longer range.
End notes: Strong focus on intelligence growth. Very effective for obtaining high lumber amounts. Despite what the tool-tip says for Hero damage in game (reduced damage to all armor types), the Alchemist deals extra damage to bosses/Hero armor; most effective damage type against Divine armor. Relatively effective in combat, despite being a support hero.
Commander: 
Powerful leading hero with multiple summon abilities and auras (Rank 15). Strength-based.
Basic stats:
85-103 Pierce damage, 650 range. Fast attack speed.
Strength: 41, +9/lv.
Agility: 34, +6/lv.
Intelligence: 26, +5/lv.
Abilities:
Battle Cry (unlocked from start)
Increases damage of nearby units [not heroes] by 300%.
Duration: 15 seconds:
Cool-down: 60 seconds.
Mana-cost: 100.
Notes: Very helpful to clear enemy waves when used on Commander's Paratroopers. I believe it goes by the units' base damages, having +100 damage, for example, from Command Aura won't be taken in consideration for the bonus.
Summon Paratrooper Squad (20 levels)
Summons 8 paratroopers at target area. They have a 3% to stun a unit for 1 second [Bash].
Duration: 110 seconds.
Cool-down: Quite long, decreases enough to allow 2 different squads to stack for the last few levels.
Mana-cost: 5.
Notes: Paratroopers' damage and HP are based off of the Commander's strength [if someone could tell me cool-down and the damage/HP gain from Commander's strength that would be nice]. The range of the ability is global, so you can help out others if they need it. Pretty good at tanking.
Command Aura (20 levels)
Damage increase: +1%/lv; max of +20%.
Notes: Not much to say, it increases damage.
Morale Aura (20 levels)
Heal: 200 +200/lv HP/second; max of 4000 HP/sec.
Cool-down: Maybe like 2 seconds between use.
Costs 5 mana to activate and 30 mana per second for upkeep. Has a small [perhaps a second or two] cool-down between activation and deactivation.
Notes: If you tome up the Commander, he's able to have Morale Aura up indefinitely. Very helpful if you choose to have Commander and his paratroops tank the end-game bosses.
End notes: Average attribute gain that doesn't help paratroopers as much as it could. Early use of paratroopers allows you to obtain early kills. Morale Aura keeps him and his paratroopers alive even longer.
Astronomer
(Rank 35). Going to be added in 6.43. More useful information will follow the release.
For any developers that read this: could I have some sort of either heads-up or clear indication on releases of what's being changed about heroes, so I don't have to go through all of them again.
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