Hero Hunters All Heroes can be viewed inside the hero menu, even before you have them unlocked. It can help out in a few different ways to see what you getting before you buy it or work towards unlocking someone. For the most part things are pretty straight forward, try to get your hands on the strongest players to advance through the most difficult parts of the game.
You may have noticed already, but the limit for star rating goes up to 10 though it seems that the highest ranked hero is a 7 star. This more than, likely means that the developers are planning to come out with updated content some time in the future. The game has been out for awhile so new characters should be around the corner and could arrive any day now.
If your trying to unlock all heroes than your going to be in for a long ride. The best thing to do in the beginning is to try and obtain the strongest team that you can. It pretty safe to say that the more stars they have the more power they will have and thus more help in battle situations. It can be a good idea to try and have at least one character from each attack type to help exploit the weaknesses of enemies.
The stronger a player is the more fragments your going to need to unlock them. One of the best ways to acquire new players is to open crates in the shop with gold. These can have a lot of strong players and you wont have to collect all their pieces to use them. Of course this can get expensive especially if your planning on spending real money. There are a couple ways to earn gold in game for free though it will take some time to save up 300 gold for the better crate.
If your interested in more related information you can check out the Hero Hunters Wiki.