Hey, there! Judging by the fact that you're reading this, you'd like to know my secret. The secret to cheating at creating depth within characters! I am here to tell you that I have found it.
You ready? Here it is.
https://www.16personalities.com/
Now it may not look like much. In fact, it looks like every other personality test out there.
But you must take it. Take the test and answer as you believe your character would. You'll find out the general personality of your character, strengths, weaknesses and it will open the door for exploring the deep, complex inner workings of that protagonist that's been floating around in your head for 6 1/2 months.
And with the result you get (Say, ENFJ.) You can look up how ENFJ's react to stress, how they deal with loss, how they build relationships. All that and find resources on those sorts of things. But just in case you just want the answers, there's also this website. https://www.psychologyjunkie.com
They have a TON of useful articles on each personality type. (Even the really rare ones, go figure!) And once you have the basic personality of your character, you can change it and alter it as you please, or just leave it the way it is! And the best part? All these resources are 100% free and open to the public.
You're welcome!