Two characteristics to define O'Brien at the beginning of Monster could be "focused" and "unemotional."
As the novel opens, O'Brien is incredibly focused. She wants to make sure that she does the best job she can. Her focused nature can be seen in how she is very direct with Stevie about what she wants from him. She talks to Steve in a very direct manner ("Let me make sure you understand what's going on.") She instructs him that he "better take this trial very, very seriously." When O'Brien insists that he just "sit there and pay attention," it is as if she does not want him to get in the way of her goal. When he asks her in a very human way if she thinks they are going to "win," she gives a terse answer about what defines a "win," and leaves. O'Brien is concerned with "her job" which is about "the law." While she wants to make him a "human being" in the eyes of the jury, it is only to feed her purpose.
O'Brien views Steve with a lack of emotion. O'Brien does not seek to forge any real connection with Steve as a human being although she wants the jury to forge a bond with Steve. When Steve tells her that he is "writing this whole thing down as a movie," she dismisses him with "Whatever." Steve is a kid, but O'Brien has no concern about his emotional condition. For her, the case is about the job at hand, the trial in front of her, and the evidence she will use to accomplish her purpose.