Thank you for the feedback! I want to create more for this, I think it's a great idea with potential. I've fixed most if not all of the bugs and am currently implementing better debugging tools for my code base as a result.
I found that vanishing characters happen when castle xp goes past the required xp and then upon buying it goes below causing the characters that arrive at certain xp levels to be unavailable again. It's unintended, I accidentally reset the wrong xp.
I absolutely agree, I am planning on improving more, for example, tips, the art and even (spoiler) new areas of the castle and characters. It does give xp for each trade, the collection doesn't though.
The main goal was to make an npc system which I achieved yay. It took most of the time to create debugging tools for and achieved a stable system to produce at faster paces. I'm really enjoying the feedback. Thanks again!