[4][5] In the same year, she starred in the movie Koinu Dan no Monogatari as the main character, Mao Morishita.

She also became a member of the subgroup ZYX along with Mari Yaguchi from Morning Musume, Erika Umeda, Maimi Yajima, Saki Shimizu, and Megumi Murakami.

[13] Berryz Kobo debuted on March 3, 2004, with the single "Anata Nashi de wa Ikite Yukenai",[14] and she was one of the lead singers of the group.

The group formed to perform the opening and ending songs for the anime Shugo Chara!

[21] On November 5, 2016, at Shinjuku ReNY, during Country Girls' second anniversary event, Tsugunaga announced that she would retire from the entertainment industry the following year to become a kindergarten teacher.

[22][6] To commemorate her graduation, a compilation CD album titled Tsugunaga Momoko Idol 15 Shuunen Kinen Album Arigatou Otomomochi (嗣永桃子 アイドル15周年記念アルバム ♡ありがとう おとももち♡, Tsugunaga Momoko Aidoru 15 Shuunen Kinen Arubamu Arigatou Otomomochi) was released on June 21, 2017, with songs selected by fan vote.

[23][24] Tsugunaga held her final concert on June 30, 2017, in an outdoor venue at Odaiba, exactly 15 years after her debut in Hello!

[27] During Berryz Kobo's later years, Tsugunaga became known as the "cutesy character" of the group and with her mannerisms and twintail hairstyle.

[2] After Berryz Kobo went on hiatus, she began playing down her "Momochi" image and stopped wearing her hair in twintails, citing that she has grown up.