When an author comes right out and describes the character's personality traits, this is called direct characterization. An example of this would be, "The girl is nice." Although easy to understand, this type of characterization doesn't always make for the most interesting characters.
More often, authors slowly reveal the personality of a character using Indirect characterization. As we read, we form opinions about characters' personalities in much the same way that we form opinions about the people around us in the real world. A character's personality is revealed through their words, actions, and interactions with other characters - it is shown, not told.
In "The Ransom of Red Chief", Red Chief's mischievous personality is given away at the very second that he is introduced as casually, "throwing rocks at a kitten on the opposite fence." As the aspiring kidnappers approach him, he "catches Bill neatly in the eye with a piece of brick." With just these two actions, Red Chief has already made it quite clear to the reader that he is not your average young boy.
We also learn a lot about Red Chief's character through his interactions with other characters. He is completely unafraid of these two strange men who have taken him away from home - rather, he is excited! In contrast, the two men who kidnapped him are desperate to escape from him! The final moments of the story perfectly illustrate Red Chief's effect on other characters; he's so exhausting that the kidnappers actually end up paying Red Chief's father to take him off their hands!