A single word for Unpretentious

How about authentic? Merriam-Webster includes the following definition:

: true to one's own personality, spirit, or character

\\ is sincere and authentic with no pretensions

which might suit your purposes?

Is modest what you're looking for? Merriam-Webster even says it's a synonym for 'unpretentious':

1 b: neither bold nor self-assertive

4 b: unpretentious

I like humble for this use.

Humble: Having or showing a modest or low estimate of one's own importance.

If you'll settle for a fixed phrase, you could say that it is down-to-earth:

1 : practical • down-to-earth traveling tips
2 : unpretentious • surprised to find the movie star so down-to-earth
_{definition from merriam-webster.com}