2007, Niigaki was placed in the unit Athena & Robikerottsu along with fellow Morning Musume member Aika Mitsui, as well as Saki Nakajima and Chisato Okai of Cute. It then was announced in July 2008 that Niigaki and Ai Takahashi would play the 70s idol J-pop duo Pink Lady in the TV drama Hitmaker Aku Yū Monogatari. From August 6 until August 25, 2008, Morning Musume performed a version of Rodgers and Hammerstein's Cinderella with the Takarazuka Revue, in which Niigaki starred as the Prince. On February 12, 2009, it was announced that Morning Musume would be performing in the United States at the 2009 Anime Expo, the nation's largest anime convention, as one of the first official guests of honor. In an interview Niigaki stated that "The movies that Disney makes are full of dreams, but when I have children, I'd like them to know about war, the pain of death and such things, so I hope they will watch [anime]." Niigaki also revealed that her favorite anime was Sailor Moon, stating that "we copied the hairstyles and had a 'Sailor Moon' playhouse in kindergarten." Niigaki became one of four members to remain in Morning Musume for seven years or more (the others being Kaori Iida, Hitomi Yoshizawa, and fellow fifth-generation member Ai Takahashi). Niigaki and Takahashi became the longest-serving members in Morning Musume history, on January 17, 2009, breaking the four-year-old record set by Kaori Iida. Later in August, Niigaki and Takahashi became the first members to remain in Morning Musume for eight years. Niigaki was later assigned to be a part of the new shuffle group ZYX-α with Koharu Kusumi, Maasa Sudo, Momoko Tsugunaga, Erika Umeda, Chinami Tokunaga, Saki Ogawa, and Ayaka Wada. On January 9, 2011, it was announced that Ai Takahashi would graduate from Morning Musume and Hello! ProjectHello! A project at the end of their autumn 2011 concert tour. Niigaki would take over the position as leader of Morning Musume after Takahashi's graduation. After Takahashi's graduation, Niigaki became the only member of Hello! Project that had joined before the Hello! Project Kids, and holds the record for the longest-serving member of Morning Musume and Hello! Project. She would graduate from Morning Musume and Hello! Project on the last day of their Spring Concert Tour, May 18, at Nippon Budokan. Niigaki was the fourth Morning Musume member to graduate at the Budokan. On May 18,