Difficult upgrading special skills
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20upgradedcharacters
If you use the same color as the hero you are feeding, you get 2% for a one-star feeder, 4% for a two-star and 6% for a 3-star. If the hero is a copy of the hero you are feeding, you get 25%.
For four- and five-star heroes, this works out in practice to having you finish raising the special skill almost certainly before you have reached maximum level. On three-stars, it’s pretty easy to get them to maximum without maxing the special skill, but you can continue to feed them, and they get 4x as much chance to increase their skill as normal (so one copy of the same hero is enough to raise it, and 10 1-star feeders of the same color is an 80% chance).
If you use another color, they get only half as much chance to increase their skill. Even though trainers give a lot of levels, they give the same chance for a special as any other hero with that number of stars.