Who is the "other woman" in this novel?

It changes over time and from situation to situation. Thoughts?

I honesty felt Kate was the other woman. David had struggled for years and had finally made the decision to have a relationship.

This is a tough question for me to answer! I agree with Lea, that it does change. At first Kate herself could be considered the other woman. The Kate David loved had disappeared and was essentially gone. After years of watching his wife, whom he loved deeply, change into someone else who wasn't really there, he found himself drawn to Jane, who at this point might be the "other woman". Then Kate unexpectedly "comes back" after the new drug treatment. Is Kate now the other woman, since David and Jane had established a relationship, thinking that Kate was truly gone and began to go public? Actually Kate is not only the other woman in the relationship at this point, she is another Kate. She's changed. She responds differently to situations and relationships. The old Kate and the new Kate were not really the same. This is complicated! ??

Yes, this is complicated.

Do you think the novelist shows how complicated?