First things first, start with the basics. The outward appearance of your character and how you want them to be seen in someone’s mind sets the tone of your book. Hair color, height, gender, age, ethnicity, etc., plays an important role. In order to be relatable your character needs a face, so to speak.
Next, tell about them. Their job, their hobbies and what makes them tick. You want to keep it brief so as to how it’s not boring, but give enough detail that they can be easily understood.
Every character should have faults, just as we do. No one is perfect and if you go out of your way to make a character that is flawless your reader will lose interest. It’s just not plausible. What makes them angry? Why do they do what they do? Anything to make them more “human” is helpful.
Body language is also very important. You can be a bitch and still carry yourself like Mother Theresa. Hand gestures, gait, mannerisms, posture: all of it is relatable when describing your character.
I find it easier to make a list in the beginning, even if my characters don’t have names yet, and simply write a few things here and there about their personalities. When you really start to dig into writing your story, the background will already be there. It also helps to sit on it for a while; let those characters mingle around in your brain for a bit. When you begin writing, those characters will have already had conversations and interactions that perhaps you hadn’t thought of before, and that can really help add to your storyline.