What is the phrase that means a quality that is "constant throughout" a larger thing

Solution 1:

You actually said it: A constant.

This phasing is common to describe a quality that doesn't change.

Throughout his life, despite changes in circumstances, hard work was a constant.

Throughout his life, the only constant was his dedication to hard work.

The question might be, was this character forced to work hard, eager to work hard, stuck working hard? What is hard work? Is it the effort you put into it, or unfortunate circumstances, such as terrible conditions, horrible bosses, physical labor? Just saying "hard work was a constant" in someone's life does not give much insight into either the work or the person.