For me, the perfect answer was a spreadsheet. I made columns for first name, last name, and notes describing who they are and their relationship to other characters. I also made a column for each book and potential book in the series, putting an ‘X’ in the column if the character appears on the pages of the book and an ‘m’ in the column if the character is simply mentioned or referenced.
Then I took the spreadsheet and sorted it by first name so when I needed to add a character, I could quickly see where in the alphabet I’m overloaded and which letters would add more diversity and fit well within the already existing cast.
Some people use a notebook, word processing document, or a database to keep track of character information. I like the at-a-glance handiness of my one-page spreadsheet.