Hiroyo Matsumoto
Ahead of Mio Shirai's impending retirement from professional wrestling, she made her Stardom debut on July 26 to wrestle what was billed as the Shirai sisters' final match together, where Io and Mio defeated Hiroyo Matsumoto and Mayu Iwatani. On September 23, Shirai and Iwatani broke the record for most successful defenses of the Goddess of Stardom Championship by defeating Dash Chisako and Sendai Sachiko for their fourth defense. On October 17, Shirai defeated Mia Yim in Covina, California, during Stardom's first American tour, to make her sixth successful defense of the Wonder of Stardom Championship. During the second event in Baldwin Park, California two days later, Shirai and Iwatani made their fifth successful defense of the Goddess of Stardom Championship against Hiroyo Matsumoto and Kellie Skater. On November 8, Shirai and Iwatani won the 2015 Goddesses of Stardom Tag Tournament, defeating Alex Lee and Kaori Yoneyama in the first round and Hiroyo Matsumoto and Santana Garrett in the final match, which also marked their sixth successful defense of the Goddess of Stardom Championship. After seven successful title defenses, Shirai lost the Wonder of Stardom Championship to Santana Garrett on November 23 in a match also contested for Garrett's NWA World Women's Championship. On December 7, "Tokyo Sports" voted Shirai the "joshi" wrestler of the year over Kairi Hojo, Meiko Satomura and Saki Akai.