Mrs. Alexander is the neighbor across the street who inadvertently helps Christopher with his detective work. She's noteworthy for a few reasons. For one, she doesn't mean to reveal to Christopher any secrets about his mother – she just can't imagine that he doesn't know she was having an affair with their next-door neighbor. Another thing we notice is that, even though she's a nice old lady, and has a dog, and offers Christopher snacks and things like that, he still completely distrusts her, and is afraid that she might try to hurt him.
Putting these two things together, we could say that Christopher imagines dangers that aren't there, and misses some that really are right in front of him. Mrs. Alexander helps us see this.