Some writers can keep track of every detail about every character without using notes. I can't do that. After finding myself having to go back through previous chapters to remember some detail about a character, I decided I needed an easier way to find the information. I now have a notebook where I keep pages for each of my main characters.
     Character sketches can be as detailed as a biography or only contain the information the writer created while writing the novel. I prefer to start a page for any character I know will most likely appear in the other books in my series. At first, this is a page with nothing more than the person's name at the top. As I write something about that character's physical appearance or history, I add that fact to the page. Some details about one person may lead to more specific information about someone else so two pages have to be updated.
     These sketches help to keep the timelines within the plot straight so they make sense. If, for example, the main character is forty years old and the story takes place in 2000, her birth year would be 1960. These two facts might seem unimportant until you write about her mother who you write was born in 1950. Was her mother only ten years old when the main character was born? Many readers might miss the mistake, others won't. Another example is the technology you create. If a character creates a vaccine called FGB292 which he discovers is a cure for AIDS, that same vaccine can't be invented later by another character and used to cure rabies unless some further explanations are offered.
     A few of my character sketches resemble family trees. If I mention a cousin or grandparent of Jake's, I create an outline of how they're related. It would be confusing to read that his cousin Bill is Aunt Rose's son and then later Bill's mother's name is Juliet. Sure, there can be two cousins named Bill or Rose can be a nickname, but those need to be either explained or the names need to be corrected. If I make a general statement about a character having a son and a daughter, I add some specific names I can use in later stories.
     You can choose to use character sketches or rely on your memory. I only suggest their use because they've helped me keep my facts straight.