[2] On December 10, 2006, she alone out of five (originally six, but Ayami Masuda dropped out) audition finalists was chosen to join Morning Musume.

An article published on December 11, 2006 revealed that Mitsui wouldn't be present in the 57th edition of Kōhaku Uta Gassen but would instead be featured in the group's 32nd single, Egao Yes Nude, which was released on February 14, 2007,[3] following her first public appearance on the last round of the Hello!

During the conference, Mitsui revealed her goal of captivating the public by means of her cheerful personality and smile, emulating fellow member Koharu Kusumi, whom she admires and strives to equal or surpass.

Following her concert debut, Mitsui was also featured in a small segment of the Oha Suta morning programme entitled I want to express my gratitude ("ありがとうを伝えたい", Arigatō o tsutaetai), which purports to show the viewers a fictionalized account of the events following her first concert appearances, dealing with Mitsui's sadness over the fact she had to part with her former classmates due to her becoming an idol.

In October 2007, Mitsui was placed in the unit Athena & Robikerottsu along with fellow Morning Musume member Risa Niigaki, as well as Saki Nakajima and Chisato Okai of Cute.

[6] Between December 15, 2010 (the graduation of Linlin) and January 2, 2011 (the addition of Mizuki Fukumura, Erina Ikuta, Riho Sayashi and Kanon Suzuki), Mitsui was the only member of Morning Musume under the age of 20.

Aika did not participate in the 2011 Morning Musume fall concert until the final day on September 30, when Ai Takahashi graduated from the group.

In October 2012, it was announced that Mitsui would be a member of the new unit GREEN FIELDS alongside Saki Shimizu and Yuka Miyazaki.

Beginning on April 6, Mitsui became a regular on the radio show "ALMA KAMINIITO Sound Map," replacing Ohno Munehiko.

From October 12 to 20, Mitsui performed in a play called Nettai Danshi 2 in Zenrosai Hall / Space Zero.