#plotting characters

Was planning a new story and came upon a new feat. How do you define a character's personality? When I define them, I feel like I'm not necessarily defining their personality, but rather just who they are. Like what they do, how they react to certain things, and how they think. But I feel like that's kind of straying from the idea of a 'personality'.

So, anyone, please, can you tell me how the hell to truly define your character's personality and not stray?

And then like what are the other different things you need to know about your character to truly make them come to life? I'm sure there's more and that personality is just one of them.