I will share some of the tools I have found to use to build characters. Different ones have worked for different books, different characters, etc. Not even all of the characters in the same book benefit from the same tools. Characters, for me, develop fully grown in my head. My job in telling their stories is to get them to reveal themselves to me. Different characters react to different stimuli.
If I try one questionnaire for instance and the character will not give me the information, then I try another one, and keeping going until the character spills the answers that I need to round them out.
The characters in the tale that this is the story of have been coming somewhat easily, except my chief antagonist, he is very shadowy, but he is a mysterious serial killer, so I guess that should be expected.