I'm planning to write a fantasy short story (or a longer story, perhaps?) about a young assassin/"a problem solver", who's been trained by a power-hungry court wizard and a royal spymaster to "take care of some things and criminals", such as corrupt guards and "troublesome" people, like political opponents. The story is mostly staying in a single city (the capital of a kingdom). The assassin is quite young (around twenty or so) and has been in training for over a decade. A while ago, I noticed one thing. How do I give this assassin a personality or depth? I don't want to make a character who's like "Yes, my master. Consider it done", a character without proper personality. I'd like to write a character who seems an unstoppable force, like Geralt of Rivia from the Witcher series, but is actually trying to make a difference. You know, don't judge a by it's cover. How do you plan your characters, or what do you think, if you're writing something like my character?