Ian McEwan's novel Atonement is told primarily through the voice of Briony Tallis, a thirteen-year-old girl who tends to be overly dramatic. Because she is inexperienced in such things, and because others who see and hear the behavior do not expect it from him and therefore don't recognize it for what it is, Paul Marshall is an apparently polished and refined young man who gets away with a horrible crime. From the beginning, Paul is a kind of paradox, seeming to be what he is not and not seeming to be what he is.
Paul is a rich, spoiled young man, and though Briony does not use the word, he is arrogant in his privilege. He is also a highly sexual man used to getting his way, another facet of his character which Briony does not understand. Paul has barely met his friend's sister, and already he has brushed up against her in a subtle but sexual way. Cecilia is not interested. Their cousin Lola, on the other hand, is a highly sexual creature, and Paul recognizes that in her right away.
When he first arrives, he offers to make everyone cocktails. This traditionally done by the host, not the guest. Even worse, he creates an unpleasant drink made from chocolate (his family business), which is not refreshing or even pleasing. Shortly after he arrives he makes his way to the nursery, of all places, and shares a limited sexual encounter with Lola--who enjoys the attention until he gets rough with her and leaves a mark. Emily, Briony's mother, misinterprets the sounds from the nursery because, again, she does not expect bad behavior from a "good boy." At dinner, Paul is impolite when he makes conversation with only one person at the table and does not seem to realize his social gaffe.
He is a man who has been raised with money and was presumably taught how to behave in polite society; however, everything he does is rather crude and gauche. The final act which Paul commits while at his friend's house is the most heinous: he rapes the young cousin Lola. It is this event which triggers the rest of the story, and Lola--who is perfectly aware of the truth--does not accuse Paul of the act, allowing Briony to make her strident but false accusation and set in motion a course of tragic events for which she spends her entire life trying to atone.
In an ironic twist, Paul and Lola marry, and on the outside they appear to be a perfect high-society couple--another sharp contrast to what they really are: rapist and victim. His character is a paradox, constructed in such a way that, though we expect a rich boy who is friends with a fine young man like Leon to be well behaved and well mannered, we are reluctant to believe the worst of him--even though he demonstrates it time after time.
I am including a general link to the eNotes study guide for this novel; if you would like to review what Paul says and does at the Tallis home, see the chapter summaries for Part I, chapters 4 (when he first arrives) to 14 (after the rape and arrest of the wrong person).