Arandor
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Actually, if I remember correctly, there's a hero named Tarnum in one of the Heroes 3 parts, but he has a different face, and he's aligned with the Tower.
So, he's mentioned in the game, but not as a super-hero.
Well, I don't know... I've completed almost all the campaigns, except for "Trial by Fire" (I'm currently playing the last map), and "Dragon Slayer" (I'll start it when I finish the previous one). Maybe he's in this campaign, since it's about the Tower (although I doubt you have to play as him; he's probably an AI character... but I'm not sure). And there's the last campaign (I don't know the name), which unlocks after completing all the others.
Be strong. Believe