So I outline very heavy before I get started.
Every now and then a character is so good that you want to divert from your original outline to include more of the character(s).
I am hitting this now in my next novel, The Sec Chief.
In my writing, I like to have fully developed secondary and even tertiary characters that create a thread to connect to the next novel. In the current project, a character that will become the main character in my NEXT project appears briefly in this book.
Her role has grown a bit. I allowed it. The outline has been modified downstream to account for the boulder that just dropped in the river.
The lesson is: Be flexible, as long as it makes the story better.
--Just don't be a dog chasing butterflies...