Who is Phyllis from Pixar's movie Up?

My friends and I watched the movie Up yesterday. We thought it was really good, but there was one thing that left us confused: Who was Phyllis? Russel said something about Phyllis not being his mom, but we were confused to who she was then. And at the end, they showed Russel's mom [I believe, since he yelled out "Hey mom" or something along the lines of that]. So I guess we know his mom isn't dead or anything...but who is PHYLLIS?