On August 22, 2015, Asuka attended WWE's NXT Takeover: Brooklyn event in Brooklyn, New York, where she was shown on camera and identified as "Kanna". On August 27, it was confirmed that she had signed with WWE a few weeks earlier. "Kanna" appeared at the September 10 NXT tapings, where she announced that her new ring name was "Asuka". Her debut appearance, which aired on September 23, ended with her being confronted by Dana Brooke and Emma, which led to her debut match on October 7 at NXT Takeover: Respect, where she defeated Brooke. After her debut, Asuka continued racking victories over various competitors, including main roster Diva Cameron. Asuka continued her feud with Brooke and Emma, which led to a match between Asuka and Emma, at NXT Takeover: London on December 16, where Asuka was once again victorious.
On the January 13, 2016, episode of NXT, Asuka competed in a number one contender's over-the-top-rope battle royal for Bayley's NXT Women's Championship, which was won by Carmella. On the February 10 episode, Asuka saved Carmella from Eva Marie and Nia Jax after her loss to NXT Women's Champion Bayley. Afterwards, Asuka had a staredown with Bayley, indicating that she wanted a shot at her title. After Asuka and Bayley defeated Marie and Jax in a tag team match on the March 16 episode, an NXT Women's Championship match between the two was made official by William Regal. On April 1 at NXT TakeOver: Dallas, Asuka defeated Bayley to win the NXT Women's Championship, becoming the first Japanese holder of the title. On June 8 at NXT Takeover: The End, she successfully defended the title against Nia Jax. After Bayley earned herself a title rematch against Asuka, tensions between the two began to rise, leading to the match on August 20 at NXT Takeover: Brooklyn II, where Asuka retained the title. On November 19 at NXT Takeover: Toronto, Asuka successfully defended the NXT Women's Championship against Mickie James.