|Also known as||Mittsī|
|Born||January 12, 1993 (1993-01-12) (age 17)|
|Associated acts||Morning Musume, Athena & Robikerottsu, Guardians 4|
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, following her first public appearance on the last round of the Hello! Project Winter 2007 Tours, ~Shūgetsu 10th Anniversary~ (～集結! 10th Anniversary～, ~Shūgetsu 10th Anniversary~?), which took place between January 27, 2007 and January 28, 2007 at Saitama Super Arena.
She also took part in Morning Musume's 2007 Spring Tour ~Sexy 8 Beat~. One week after the results of Happy 8 Ki Audition were announced, Mitsui was featured in a small segment of Hello! Morning which included footage from a press conference given on the eleventh. During the conference, Mitsui revealed her goal of captivating the public by means of her cheerful personality and smile, emulating fellow member Kusumi Koharu, 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.
Her first appearance at a non-Hello! Project related event took place on February 8, 2007, on Utaban, as part of Morning Musume. In October 2007, Mitsui was placed in the unit Athena & Robikerottsu along with fellow Morning Musume member Niigaki Risa, as well as Nakajima Saki and Okai Chisato of °C-ute. On the November 25, 2007 episode of Haromoni@, Mitsui got the seiyū role of Gurossan in the anime Kirarin Revolution starring Kirari Tsukishima, voiced by Kusumi Koharu. On April 6, 2008, Hello! Project announced that that Mitsui would not be able to perform in the 2008 spring tour due to her being in very poor health. Later in the week, it was discovered that she had suffered from acute appendicitis, and would be out of action for two more weeks. She recovered and continued with her scheduled activities. On August 8th, Morning Musume collaborated with female theatre group Takarazuka to perform Cinderella the Musical by Rodgers and Hammerstein. Mitsui took on the role of a fairy, a town girl and a princess.
In 2009, it was announced that Morning Musume will be attending the annual anime convention, the Anime Expo in Los Angeles, California as Guests of Honor in July. All members of the current lineup including Mitsui Aika attended the event. Morning Musume held autograph sessions and a concert. The number of fans that attended the concert reached over 7,000 people.
Mitsui currently has a blog only available to Official MM Fanclub members called "Pastel Colour Days".
- Name: Mitsui Aika (光井愛佳)
- Nickname: Mittsi
- Birth Date: 1993-01-12
- Birthplace: Shiga Prefecture, Japan
- Blood Type: O
- Height: 156.4 cm
- Favorite Food: Plums, Strawberries, Roast Beef, Kombu, Surume (Dried shredded squid)
- Disliked Food: Chocolate
- Favorite Color: Black, Light Purple, Gold
- Favorite Phrase: "Hakuna Matata"
- Special Skill: Bowling
- Audition Songs: Blue Bird by Hamasaki Ayumi, Furusato, Osaka Koi no Uta
- Hello! Project Groups:
- Concert Units:
- Wonderful Hearts (2008–)
For discography as part of groups, see Morning Musume#Discography, Athena & Robikerottsu#Discography, Guardians 4#Discography.
-  ハロー! モーニング (Hello! Morning)
- [2007–2008] ハロモニ@ (Haromoni@)
-  ベリキュー! (Berikyuu!)
- [2008–2009] よろセン! (Yorosen!)
-  きらりん☆レボリューション (Kirarin☆Revolution) (guest role)
- [2007–2008] Morning Musume Mitsui Aika no Ai say hello (e-radio)
-  シンデレラ The ミュージカル (Cinderella the Musical)
- Morning Musume Happy 8ki Audition songs:
- Round 1: "BLUE BIRD" (Hamasaki Ayumi)
- Round 2: "Furusato" (Morning Musume)
- Round 3: "Osaka Koi no Uta" (Morning Musume)
- She became good friends with Yoshizawa Hitomi, who adopted an "older sister" figure.
- Proclaimed herself to be Kusumi Koharu's new rival.
- Youngest member of Morning Musume.
- Her favorite number is 397 because it was her entry number in the audition. Her mother stated she won because it means bloom like a flower (sa ku na = 397).
- Has a toy poodle named Leo. Recently took home a Siberian Husky, which is proclaimed to be one of her dream pets.
- Lives with her older sister in Tokyo.
- Her grandmother named her.
- Has an inhaler with her in concerts in case her breathing gets bad.
- Noticed to be wearing flat shoes for single costumes. One cause of not wearing high heels is rumoured to be the operation she had earlier, possibly affecting her ability to dance with high heeled shoes.