How To Create Great Characters

ii. Personality

This is possibly the most important part of your character, and it's what defines them. You want it to be three dimensional, realistic and relatable to an extent. You don't want a Mary Sue. It's crucial you get this right.

When developing a personality for a character, I like to think of the overall archetype. Keep in mind that good chapters don't stick to one archetype and combine multiple, with some more prominent than others. For example, I could use the archetype of "the sage" for my character - that is, a wise and knowledgeable person. If I only used one, this character would be pretty boring. So, let's throw in another common archetype: "the jester." This joking side might not be as prominent as the side of wisdom, but it's there and it adds another dimension to the character. Let's add a third, for good measure - "the hero."
Now, we've got a knowledgeable and heroic character, who's got a penchant for wisecracks. Looking good!

But that's not all. Of course, every good character needs a mix of good and bad traits. Yes, this joking sage may be the most heroic of them all, but he might be stubborn to a fault, or arrogant. Maybe he's snappy or grumpy when things don't go his way. You want to balance out the good traits with the bad traits. "Nice but can be mean sometimes" is not a personality.
Here's another way I like to figure out individual traits. I'll make two lists - one of good traits and one of bad. I'll go down each list and pick five from each category that fit the archetypes I want my character to fit. Sometimes, I even do this before even considering archetypes, instead just randomly selecting a couple and creating a personality around them.

When I'm satisfied with all of this, I either like to make a bullet list of everything or write it up in a lovely articulate paragraph. It really just depends on how lazy I feel, and how much detail I've put in.

Here's something interesting for you though: even though this is meant to be the concepts stage with only rough outlines, personality is one of the few things where I personally like to get all the details out of the way first. You might prefer to make a quick personality concept (eg. A wise, yet grumpy and sarcastic old guy with a heart of gold) and work out the details later. This is just my preferred method, so I'd suggest trying both and seeing what you like better.