While there is no comprehensive list of all of the heroes available, you can definitely assume that no character within the whole of the DragonBall universe is off limits.
There are four main types of heroes in Z Fighters:
S Rank Heroes
A Rank Heroes
B Rank Heroes
C Rank Heroes
Obtaining Heroes Edit
There are two major ways to obtain heroes: reviving and calling.
When you gain enough souls, it becomes possible to obtain that hero depending on their rank.
S Rank takes 30 souls.
A Rank takes 15 souls.
B Rank takes 5 souls.
C Rank takes 1 soul.
It is possible once you have reached an appropriate level to gather "soul orbs" and use Babidi's Page to summon souls; this is based off of the characters you have in your inventory. Depending on the level of the soul you are trying to summon will depend on how many soul orbs are required to generate 1 soul. Obviously, the higher level the soul (i.e. SS2 Gohan) the more orbs it takes to summon just one of his soul.
Their are three types of calls:
Epic Call: Possible to obtain S and A Rank heroes. Gain a free try every 72 hours after use, purchase 1 hero with 300 gammets, or do a mass draw of 10 heroes (with a guaranteed S Rank) for 3000 gammets.
Super Call: Possible to obtain S, A, and B rank heroes. Gain a free try every 24 hours after use or with 100 gammets.
Basic Call: Possible to obtain A, B, and C rank heroes. Gain 5 free tries every day with a 10 minute cooldown or with 10 gammets.
Normally Epic Calls end up in A Rank heroes, Super Calls end up with B Rank Heroes, and Basic Calls end up with C Rank Heroes. I find it more effective to buy Epic Calls rather than the other two.