What effect does willpower have in the game, or what is it's definition?

There is a link over at Steam - http://steamcommunity.com/app/246760/discussions/0/276237094311978942/ - where the dev explains it.

Talking of which, I wish the dev would come over to the GOG forums as often as he does the Steam ones :( Especially as they have just released a new game here you'd think they might like to show a bit of interest with what is going on with the GOG version....

I'd second that especially as a) he's very good at explaining why things work in a certain way or even just hints and tips and b) the game hasn't been that successful and a bit of presence on the GIG forums might help a few sales