Hi there! This is the section where you can figure out who your characters are, which will help you figure out what your plot is. The official snowflake method has you painting the broad strokes of your characters at step three, and then detailing them out after you figure out a bit more of the plot. Since characters are all so different, this doc will not have any set restrictions to how you figure out how they are, however I will give you some suggestions.
Make a new section for each main character, so that you can expand it as much you need to without restrictions.
Some good questions to ask yourself for painting the broad strokes of your characters are: What are these character's motivations? What are their goals? What are the things keeping them from achieving their goals? What will be the epiphany that will help them achieve those goals?
At some point, you may want to create a table that holds all your main and side characters, to keep track of everyone's basic information like name and hair color. Believe me, you will thank yourself later on, when you can't for the life of you find what that one guy was called that you mentioned once somewhere chapters and chapters ago. You can keep it simple (don't even bother to put anything but a name for the characters you only mention once or twice), or put literally everything in there. I have created a blueprint for a basic table at the bottom of this page that you can use as a starting point. (Bonus tip: press command-k when clicked into the character's name field and find the name of that character's page to link it up. This will make this table into a nice little directory to all your different character pages.)
One exercise from the snowflake method which can prove to be extremely useful is to outline each character's individual journey. This can be as complex as it needs to be, but the main things to cover are: What information do they know at different points in the plot? What are they up to? This can both help to make your book's world feel more real and fledged out, and also help work out obvious flaws that you may have missed while just working on your main plot.