Not really a free game
Technically, if given enough time, you can play this game for free.
However the reality of it is you won't get far very fast without spending cash ... and a hell of a lot of it, just look at the "DLC" which is really just buying in game currency. This post is based off the mobile version of the game.
1) You use Iso-8 to "Level Up" your heroes. At first it isn't so bad, a hundred to a couple hundred, but as you get up to the mid-levels you'll be needing over 1000 to gain a single level on a single hero. Your 'party' consists of three heros.
Iso-8 is won from doing the story missions where you will get either 100, 250, *or* 500 as a reward. From the main story you can get roughly 257 rewards in total, and about 1/3 to 1/2 of them will be one of the above iso-8 rewards. The remainder will be "Hero Covers", one time use items, or "Hero Points".
Repeating a story mission after you get all the rewards will give you 20 iso-8. Doing PvP will give you (randomly) 70 or 140 iso-8. You do not get to pick a specific opponent, you are given a random set of 5 people you can play against.
2) The max level of your hero is limited by the level of the heroes powers. In order to increase a heroes powers you must either find a "Hero Cover" for that specific power, or spend 500 Hero Points. However, you can only opt to spend the 500 Hero Points after you have found at least one Hero Cover for that specific power.
Some you will get as specific mission rewards, the rest you will get by randomly 'buying' a hero cover in the store. You can get basic 1 star heroes by spending 500 iso-8 with an extremely small chance of getting a higher quality hero or you can spend 300+ Hero Points to buy heroes at a minimum of 2 star. Sometimes you can buy specific hero covers, but they cost 1000+ Hero Points.
3) Hero Points. These are the hard to get currency. You'll get ~2000 to 2500 playing through the entire game, plus counting ones you win in PvP Events. This is what a lot of people blow real money on buying.
4) PvP. After finishing the story of the game (which is impossible if playing free btw) all that is left is PvP. However, because of the Pay-to-Win mechanics of the game you will quickly find yourself grossly outclassed unless you also shell out cash to upgrade your heroes.
Hero power is greatly influenced by their star rating. The basic heroes you start with, and most of what you find throughout the story, are all 1-star. You will get a few 2-star though and you might get lucky with a couple 3-star. If you will the lottery, you'll get a 4-star. Not only are higher star heroes more powerful, but they also start at a higher level and have a higher maximum level. For example:
A 1-star starts at level 1 and has a max of level 50.
A 2-star starts at level 6 and has a max of 85.
A 3-star starts at level 15 and has a max of 115.
A 4-star starts at level 30 and has a max of 230.
So as you can see, unless you are going to shell out real cash to buy hero points to recruit those higher star heroes, you will not stand even the smallest chance in PvP.
5) So in #4 above I said the main story is impossible when playing free. Well, that's not technically true, but it does require (months) of time to beat it. The final boss is level 115. My heroes as a free player are level 57 (2-star hero), 45 (2-star hero), and 38 (3-star hero). To level up my level 57 I need 1117 iso-8. So I would have to win 8-16 PvP matches in order to level just one of my heroes a single level. And even if I max him out, he'll still only be level 85 to the bosses 115. I have a couple other 1-star heroes leveled up that I had started the game with but they are now for all intents and purposes useless.
6) So what happens if you lose a match? Nothing much, you don't lose anything. Except time. Whenever your heroes take damage they heal a fixed amount of hp per minute of real time. So how much time? Well, my level 57 takes over 4 hours real time to heal up. Unless, and you guessed it, you spend real world money on a health pack to instantly heal up! Even if you don't lose the match, your heroes still have to heal up whatever damage they took. You can continue to play without them being fully healed, but you risk them getting killed and the loss of a single hero can seriously hinder your ability to ultimately still win the match.
So the above issues are why this game is extremely punishing to play as a free player, yet on the other hand why it requires such a huge monetary investment of cash to actually be competitive. I would consider this game to be a perfect example of free to play done poorly. It is *extremely* pay-to-win based.
Just thought I'd write this mini-reivew so you know what you're getting into with this, and I'd strongly suggest not paying into the game, or you'll quickly find your wallet empty and still find yourself facing opponents with much higher level heroes than yourself.