LDS believe that we not only have a spiritual father (God the Father, Heavenly Father) but we also have a spiritual mother (Heavenly Mother). No one knows who this is for sure, but it is not the Holy Spirit. The Holy Spirit is the third person of the Godhead and is neither male nor female. There is no name given for Heavenly Mother, it is possible for it to be Mary, but is not neccessarily her. We won't know until we reach the afterlife and meet her.