Having a Who is Who document can really help keep your characters and important details straight when you are writing a series of books.
My Who is Who document began as a MS Word document that I started probably after the third book in my series because I noticed I was mixing up relevant information. NOTE: Do not be like me. Start your Who is Who sheet now before you accidently publish errors in your ebooks! Now I have evolved into a more complicated Excel sheet as my imaginary family has grown.
In this document I note important details like a genealogy of my characters. I document parents, siblings, children, cousins, friends, etc. From there I build out on secondary characters to know who has come in contact with who. A brief description of the scene where characters know each other, often in bullet points can be very helpful.
I also include brief physical descriptions, any odd characteristics, professions, or items that I need to remember that would relate to the character.
I divide my characters by series, even though I do have some cross over of characters. This helps me to remember who belongs in what series universe.
Here is an example of one of my Who is Who sheets:
Check out the video where I discuss my Who is Who document on YouTube: