The word unassuming means modest, lacking in arrogance, pleasant, or polite. You'll find that some of the most unassuming people are actually the most interesting and powerful of all. They're just decent enough not to display it all the time.
When you assume, you draw conclusions that you shouldn't. If you're unassuming, you don't make that mistake. Even though he was a rock star, I found Jason to be unassuming and delightful. He treated everyone like a friend. It's the height of irony that the real Wizard of Oz turns out to be an unassuming country gentleman, when the image he projected was of fearsome, raw, tyrannical power.