What's the point of introducing a new girl to the harem if we already know she doesn't win? We saw the future like 70 chapters ago and he confirmed that he had the kid with one of the original five because of the picture.
And she still hasn't picked up on the not-actually-dojo-ness?
I guess with this harem's protag being as sharp as he is, the helping of obliviousness had to go to some character.