Who is Mike/Ben from Before I Go to Sleep: A Novel and what is their importance?

Mike is referred to as Ben during the bulk of the novel because he has told Christine that he is her husband Ben. Mike is a man with whom Christine was having an affair in the months before her attack in a hotel in Brighton. A few weeks before her attack, Christine ended her affair with Mike, but he refused to believe she no longer wanted him. Mike lured Christine to a hotel in Brighton and attacked her, leaving her for dead. Christine survived the attack, but suffered such devastating head injuries that she can no longer retain memories for a significant amount of time.