Heroes aren't perfect, but they try to do the very best they can for their Gods. The heroes represent their god, being either evil or angelic, being weak or strong; though all heroes are brainless, otherwise they wouldn't do so many stupid, mindless, idiotic things. Although some of the things heroes do are quite heroic, surprisingly.
The characteristics of a hero depend all on their god. If their god is nice, kind and benevolent, then their personality will end up the same. And it will be the opposite if their god is the opposite. This comes from their god constantly insulting them and punishing them, even when they have done to nothing offend their mighty one.
Heroes write down everything they do in their diary. The little brainless wonders will write things down even in the middle of battle. They record when they kill a monster, sell something, buy something, go get drunk, et cetera.
The diary is mainly for their god (you) to check up on them and keep up with their day-to-day lives. All of the diary entries are listed in chronological order, so you know exactly what your hero did whenever they did it.
Additionally, once the diary is tattered and worn, the hero will buy another one, and the diary events will be reset. And if a creature tears a page out of the diary, it will be reset. A hero's diary is very entertaining.