What Are Secondary Characters? How To Create And Develop Them
Sarah Bonner, jerichowriters.com

Your Guide To Creating Secondary Characters
NY Book Editors Blog

How to Write Supporting Characters (Masterclass)
Margaret Atwood (Masterclass)

30 Days of Tips for Developing Characters: Side Characters
Jean Veillette Bowerman, Script Magazine

5 Archetypes for Supporting Characters
Gabriela Pereira​, DIY MFA

Secondary Characters: A Complete Guide to Understanding and Creating Supporting Characters 
Novel-Software.com Blog