Shirai made her first official appearance for the promotion on June 30, 2018, at the WWE house show in Ryōgoku Kokugikan, where it was announced she had signed with the promotion and would be competing in their developmental territory, NXT. In July, it was announced that Shirai will participate in the second Mae Young Classic tournament. On August 8, Shirai defeated Xia Brookside in the tournament's first round in her WWE debut match. The following day, on August 9, Shirai defeated Zeuxis in the second round, Deonna Purrazzo in the quarterfinals and Rhea Ripley in the semi–finals to advance to the finals of the tournament, that took place at WWE Evolution, on October 28, where Shirai was defeated by Toni Storm.