Most of the plot revolves around Wilson and Jane, and Noah and Allie as mirroring characters that reflect Wilson and Jane's problem and foreshadow a solution.
Wilson and Jane Lewis are a couple who have been married 30 years by the time Wilson narrates the story. He is narrating in past tense explaining how the year before, for their 29th anniversary, he forgot the ocasion, and then tried to make it up lamely.
It is because of this that Jane and Wilson realize the emptiness in their relationship, and what prompts Wilson to use his notes and previous experiences to fix his marriage.
Similarly, Jane and her sisters go back in time to the time when her parents, Noah and Allie, were married (in the story "The Notebook"). It has been 5 years since Allie died, Noah is deeply depressed and has now decided that Allie has reincarnated into a swan by a pond near the house.
Yet, the importance of this complex conundrum is that Allie is a very much real character in the story. Allie is Noah's deceased wife. She does not formally appear in the novel, but most of Noah's problem is directly linked to her death. He continuously mentions her throughout the story, and even his doctor, Dr. Barnwell, has to treat him considering the reality of Allie as a very present entity in his life.
Conclusively, like stated previously, Wilson and Jane are both the main characters living a parallel conundrum as Jane's parents Allie and Noah, in that both couples within their circumstances were forced to command their inner strengths to ensure that their relationships endure against the odds.