The hero can be found by killing the enemy on the map. The higher the level of the hero, the higher the chance of finding a hero.
Example: if you have the 2 weakest heroes, you will need to raise the level of the second hero to at least level 3 to find the third hero.

Odds of finding by hero level:
Level 3 - 1.4%
Level 4 - 3.3%
> Level 4 - 10%
1 1
100
5 5
400
25 25
1800
100 85
8000
350 325
30000
850 1000
90000
3000 3500
320000
10000 10000
1000000