Hello Mighty Mom, great question!
Personally, the term "believer" by definition implies the behavior of belief or faith in some intangible.
To non-believers, faith is a squishy term. I like the definition used in Hebrews chapter 11, verse 1: "Faith is being sure of what we hope for and certain of what we do not see."
That assurance of the unseen constitutes the behavior of a believer. I believe that I’ve answered the question to the best of my ability ;-)