hmm tough question. My characters aren’t that complicated or unique in design but they’re easy to memorize with their colors, height difference and attitude. But what helps best is drawing them frequently until your hand can draw them without effort.

For example, Jad, the father of of one of the two main characters only showed up in a chapter or two in +30 chapters, so when the story came to a point where he’s gonna become a regular, I found it easier to draw a sheet of his head from all angles (now I understood why animators do that) and kept it as reference until I didn’t need it anymore (to the point I can’t find it now to take a photo to show you).

Here is an example of another character that I had to design and get used to drawing at a rush,

I drew here from all angles keeping in mind the shape and proportions of her head. And then whenever I needed to draw her in a scene, I just used the head angle in the reference sheet.

That way, the character would look consistent and with attitude, people will know who the character is. Of course there will be change in your style along the years, but the main characteristics and features of the character will stay the same and stick to the characters and people’s heads.