"Forgettable characters" refers to fictional individuals in a story who lack distinct traits or characteristics, making them unremarkable or easily forgotten by the audience or readers. Full definition
Even the most forgettable characters return and a half - dozen more are introduced, to the point that it's impossible to remember how everyone met. (chron.com)
Just because you have more forgettable characters and more of a bland plot doesn't automatically make it any better; it just means you have more of it. (gaming-age.com)