When we started writing our first book, because there weren't a lot of characters we were able to remember them. When we decided to continue the book into others and create a series, we started having to keep track of all of the characters. So we kept notebooks on the different ideas we had, the different characters we were planning on putting in the books, what time era we were going to write about.
I put up in my office a poster board that listed the twelve islands, who was in charge of each island, the island theme and time period and any characters that came from that island. As we wrote the books and new characters would be introduced we put them under the correct island. We did this so that we wouldn't mix up characters and use different names for the same characters and it made it so their history stayed the same.
Consistency is important aspect of writing. If you have a character that is in the car and then all of a sudden they are in the house, you need to explain how they got there. Your writing needs to flow. Which means there shouldn't be parts where people ask themselves, okay how did that happen because it will stop the flow of the book and people will be more likely to put the book down at that point and might not pick it back up.
I used to play online games and in this game we were able to create areas where people could go into and fight monsters. So I had created an area and in the first part of the areas was the entrance to the Crystal Castle, and it described the area as being bright and filled with rainbows and light because the sun was reflecting off of the crystal. It sounded great until I went back and reviewed it before it was released and realized that it sounded great but the room was listed as a dark room without light, so how could the sun reflect off of the crystal if there wasn't a light. This is the kind of thing you need to be mindful of because it could completely through your writing our of focus and make it difficult for someone to relate to.
So remember, it is always important to make sure your characters have consistency in their behavior and history.
As always, have fun with your writing.