A character arc is not a mood chart. It is a belief that is tested, defended, and eventually either abandoned or paid for. The plot exists to apply pressure to that belief; when people say a story feels episodic, they usually mean the events are not touching anything the character holds.
Three shapes cover most of it. The positive arc, where a false belief is given up. The negative arc, where it is kept and the cost is paid. And the flat arc, where the character does not change and the world around them does, which is the shape of most detective fiction and of nearly every effective founder story.
For brand and testimonial work the flat arc is usually the honest one. A customer who fundamentally changed as a person because of a software purchase is a story nobody believes; a customer who was right about something and finally had the means to act on it is one they do.
What is a character arc?
The change in what a character believes, caused by the pressure the story applies to that belief. Positive arcs give up a false belief, negative arcs keep it and pay, flat arcs hold and change the world instead.
Does every character need an arc?
No. Flat arcs are legitimate and often more truthful in commercial work, where claiming a customer was transformed by a product strains credibility. What every character needs is a belief the story tests.