Usage patterns
〈人〉は〈事〉に臆病だsomeone is timid/fearful about something
He's timid about talking to people he meets for the first time.
臆病な性格a cowardly/timid personality
She has a timid personality and finds it hard to try new things.
〈人〉が臆病になるfor someone to become timid/cowardly
Once you fail once, you tend to become timid.