Geralt (from The Witcher) is actually a guy named Jerry who is playing an elaborate LARP. All the combat, effects from potions and magic attacks are what he imagines would happen, but all he's actually doing is yelling random things at the other people playing.
Isaac Clarke (from Dead Space) is an evil necromancer. The necromorphs aren't the result of the Marker; it's entirely Isaac's fault. He sacrificed his girlfriend to bring the dead back to life, but now her soul is haunting him. He's merely trying to regain control of the situation and his undead minions so he can conquer the universe.
Diddy Kong is actually Donkey Kong's illegitimate son and doesn't know it. Donkey Kong does, though, and he's trying to make amends for abandoning Diddy and his mother, Doddy, when Diddy was born.