I think choir/dark church events increase willpower, and also likely a class or two, though I would have to double check the code for all of them. Stats are gained from classes (each class adds 1 stat), clubs (gardening club, choir/dark church, book club (gives mana regen!), and jobs (kitchen, tavern, and nurses)
It's somewhat logical in most cases (you'll get intelligence and mana from stuff like book club or magic classes, or strength and agility and hp from more physical jobs and clubs like kitchen work or combat courses, etc etc) Some of them can be found on the wiki here, although its not 100% up-to-date with everything.
https://hmadarktower.com/wiki/index.php/Main_Page
Most consistent way to get willpower is probably attending choir or repeating the mana manipulation 1 class (which gets you +1 each and is available at the start)