In 1999, the Hello! Project group Morning Musume was holding its third generation audition titled Morning Musume 2nd Tsuika Audition and upon the recommendation of a friend, Sonim participated but ultimately was not chosen for the group. Following the conclusion of the audition, she was scouted by Fukuda Asuka's manager and Harmony Promotion president Wada Kaoru.
In January 2000, Sonim and Goto Yuki, the brother of then Morning Musume member Goto Maki, formed the group EE JUMP produced by Tsunku, managed by Wada Kaoru and they released their debut single, "LOVE IS ENERGY!" on October 18, 2000.
In August 2001, Goto took a break from the group following an altercation with their manager, and Sonim recorded the group's fifth single titled "WINTER -Samui Kisetsu no Monogatari-" as a solo, released on November 28, 2001 and credited as "EE JUMP featuring Sonim". On April 12, 2002 FRIDAY magazine published photos of Goto at a hostess club and reported that he was drinking underage, and as a result the group announced their disbandment, and their first album which was scheduled for release on May 9 was cancelled.
On August 21, 2002 she released her debut single as a soloist titled "Curry Rice no Onna".
In November, Sonim collaborated with Osawa Akane to form the unit Tomboy, and they released their debut single "Superstar" on November 14, 2007 under Avex Trax.
- Birth Name: Sung Sun-Im (성선임)
- Stage Name: Sonim (ソニン)
- Birthdate: March 10, 1983
- Birthplace: Kochi, Japan
- Ethnicity: Korean
- Bloodtype: O
- Height: 159cm
- Western Zodiac: Pisces
- Eastern Zodiac: Pig (Boar in Japan)
- [2003.05.14] Hana (華)
- [2002.08.21] Curry Rice no Onna (カレーライスの女)
- [2002.11.20] Tsugaru Kaikyou no Onna (津軽海峡の女)
- [2003.04.16] Tokyo Midnight Loneliness (東京ミッドナイト ロンリネス)
- [2003.10.08] Goukon Ato no Famiresu Nite (合コン後のファミレスにて)
- [2004.02.11] Hontou wa ne. (ほんとはね。)
- [2004.09.15] Jigsaw Puzzle (ジグソーパズル)
- [2005.01.13] Asunaro Ginga (あすなろ銀河)
- [2005.09.14] Chiisana Yoru Tabi (小さな夜旅)
- [2003.07.25] Sonim Mania (ソニンまにあ)
- [2004.06.10] Gekkan Sonim (月刊ソニン)
- [2006.02] digi+KISHIN girls Sonim (digi+KISHIN girls ソニン)
- [2007-2011] Sweeney Todd (as Johanna)
- [2008-2009] Miss Saigon (as Kim)
-  Henry VI (as Joan la Pucelle (Joan of Arc))
-  RENT- Mimi (as Maureen (2012-2015))
-  The Threepenny Opera (as Polly Peachum)
-  Mozart (as Constanze)
-  Wuthering Heights (as Isabella Linton)
- [2015-2016] Dance of The Vampires (as Magda)
-  Kinky Boots (as Lauren)
- [2016-2018] 1789: Les Amants de la Bastille (as Solene Rigot)
- [2017-2020] Beautiful - The Carole King Musical (as Cynthia Weil)
-  The Rocky Horror Show (as Janet Weiss)
- [2018-2021] Marie Antoinette (as Margrid Arnaud)
- When EE JUMP was formed in 2000, the group and its members were signed to Harmony Promotion which was created by UP-FRONT AGENCY in 1994.
- As a solo artist, her first four singles and first album were produced and written by Tsunku.
- She was one of the few talents of Korean descent under UP-FRONT AGENCY, the others being Shim Min and Jang Dayeon.
- She owned a male chihuahua called Kan (感) that passed away on November 15, 2018.