Heroes are somewhat like special recruits that you do not have to pay for in the game. Whenever they die in battle, you may summon them again for free, after a penalty time is served. Each of the heroes available in the game have their own unique skills that can be useful in different situations. They also differ in terms of durability in battle, speed, and range.
Not much information is handed out in the game about heroes. For one thing, 3 heroes are initially available for you to us for free, and the remaining 6 are to purchased separately and individually. We have yet to find out if these (paid) heroes can be obtained as you play the game too. There are also vacant slots that probably suggests room for new heroes in future update of the game. Nonetheless, we'll list all the heroes that are currently available in the game, their skills, and their passive benefits.
Each time a hero is used in a mission, it accumulates experience points that would earn it a level. Each time a hero levels up, he/she earns 3 points of upgrade in his/her skill gauge. Each hero has 2 unique skills, and one passive skill. There are two tiers to reach for in each of the gauges -- earned every time you reach five points of upgrade in a skill. The first tier is marked like that of a sergeant badge, and the 2nd tier (after further upgrading it 5 more times) marked like a 1-star general status. It is also helpful to know that you may re-appropriate the points of upgrade in every hero at any point in time.
There are two different types of skills in every hero. The first one is like a special action/command that it performs every time you activate it in the battlefield. Every hero usually has two special action/command in the game. Every time these actions are used, they start a cool-down sequence wherein you are not allowed to use that particular action until its gauge fills up again. Different skills have different cool-down duration as well. The greater the effect of the skill is, the more likely slower (longer) the cool-down duration gets. On the other hand, you also have one passive skill for every hero. Passive skills range from those that cuts down the hero's respawn time, increasing the range, walk speed, and stuff that usually buffs them.