Nothing about j = √−1 is fictional in an AC circuit. It's a bookkeeping trick that lets one algebraic object carry two real numbers — amplitude and phase — so engineers can stop solving differential equations and start doing algebra.
The name "imaginary number" is one of the worst-marketed ideas in all of mathematics. It was coined centuries ago as an insult by mathematicians who thought numbers like √−1 couldn't possibly correspond to anything real — and the name stuck long after the math proved otherwise. In electrical engineering, a complex number a + jb (engineers write j instead of iso it's never confused with current) isn't standing in for something unmeasurable. It's a compact way to carry two real, measurable numbers — a magnitude and a direction — as a single object you can add, multiply, and divide, instead of juggling two separate numbers and a page of trigonometric identities to combine them.