The adjective unpretentious is perfect for describing someone you know who's modest and natural and never tries to impress other people.
Pretentious comes from the Latin word meaning "pretend," prætensus, so when you're unpretentious, you see no reason to pretend. Some of your favorite people are probably unpretentious, tending to be honest and genuine. When extremely talented or well-known people are unpretentious, they feel no need to show off or remind you of their accomplishments. A place can also be described as unpretentious, if it's simple and comfortable, like the cozy, unpretentious cafe where you like to hang out.