In Mull's novel, Sky Raiders, semblances are people that look real, but aren't. Most of them don't know they aren't real.
Here's an excerpt where the protagonist, Cole, questions a gladiator-type warrior, Lyrus, after he puts a cloak around him that helps Lyrus see himself for what he really is:
Cole blinked. "You know you're a semblance?"
"Not until you gave me the cloak. It freed me to know what I had to know in order to serve you. Whoever made me caused me to ignore my true nature. You were helping me catch glimpses, but now I see plainly. I didn't realize I had been fabricated. This is common with semblances. We play a role without much self-reflection. It helps us seem more authentic."
When I read that the first time, I was stunned. Such truth! Such wisdom! And to find it in a middle school book? I love it.
So, from now on, when I meet someone who lives their life by playing 'a role without much self-reflection,'...I'll know to do what Cole does: use my magical Jumping Sword to get as far away from them as possible.