Who is Claire in Before I Go to Sleep: A Novel by S. J. Watson?

Claire was Christine's best friend in college. After a memories of Claire urface, Dr. Nash requests her to visit Christine in the hospital. From Claire, Christine learns that she had an affair during her marriage, and shortly before her attack. Claire also reveals that she had an affair with Christine's husband while she was recovering. Shockingly, Claire is the first to realize that Ben (Mike) isn't who he claims to be, and when Christine is in danger..... it is Claire who helps with her rescue,