Usage patterns
内気な性格a shy, reserved personality
He has a shy personality and isn't good at speaking in front of people.
内気を克服するto overcome one's shyness
I joined the drama club to overcome my shyness.
〈人〉は内気で〜being shy, (someone does/doesn't do something)
She's shy, so she has a hard time talking to people she's just met.
内気なところがあるto have a shy side
He has a shy side, but once you get close to him he's a great talker.