Personal quirks helps make a character feel real. Everyone has something that's funny and unique that they like, for example: having a tradition they follow religiously, strongly disliking something that makes no sense to dislike, something odd that is extremely liked by this character, etc...
That's one of many examples of making your character feel real and immersive.
You can also have more detailed backstory to said character slowly revealed, reasons for morals, tragedy in their history, contradicting emotions...
Overall, put a lot of thought and detail where if you sat down with your character and asked them a set of questions, and you know how they'll respond each time and be consistent, and their personality of how they respond is also in the interview with them (do they divert the question to something else, do they make eye contact with you, how thorough are they with speaking.)
Just, a lot can be done lol.