I'm trying to make a character, and I'm first creating their world, but I'm can't figure out whether I want it to be all magical or more realistic. Helluva Boss did this great. It took place in hell, and showed us the demons of hell. Another example is Death Note, it is realistic but has some elements of fantasy, mainly the death note itself. Jujutsu Kaisen, and Persona have lots of fantasy or horror but still makes it realistic. Unfortunately when I make my world I'm not sure why but it feels a little unrealistic. Let me give another example: Identity V. Identity V basically is about a detective trying to find a girl, but things went more deep as he progresses, and this is where all the magical stuff takes place.
I don't understand what makes it realistic but other things are still fantasy or magical. I asked discord writers, and they said that its the character, not the world-building. but I still need more advice about this. Any comments?