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Letos starts with only one ability, "Dark Magic," while almost all other heroes have two, such as "Enhanced Chaos" or "Attack" + "Regular Chaos," etc. This means he will always be one level behind practically any other hero, all other things being equal.
Kifrah is good in any case; Leadership is a significant advantage. Of course, Minotaurs are slow, and if you're playing against a Dark Mage, they will likely manage to charm the bull-headed creatures or cast Berserk right before their turn, thus turning Kifrah's advantage to their own benefit.
Erin has a useful skill, as she is supported by witches when attacking with magic. This works very well against wizards because the mirror won't work with area-of-effect spells, and if you use area-of-effect magic to target an enemy unit in the center, the witches will also attack it. The witches themselves are the weakest part of the Dark Elf army, but when playing as Erin, they become a very good complementary unit in her elven army. At the beginning, Erin has excellent skills – attack + chaos (while Letos only has one skill – boo-hoo-hoo-hoo).