Question: Sechskies is a six member South Korean boy band that includes what South Korean rapper whose first solo single was "A-Ha"?

Context paragraphs:
ZE:A discography: This is the discography of 9-member South Korean boy band (also known as (Children of Empire, Korean: 제국의 아이들 ).

Supernova (South Korean band): Supernova (hangul: 초신성) is a South Korean boy band formed under Mnet Media in 2006.  The group composed of six member: Jung Yunhak, Kim Sungje, Kim Kwangsoo, Yoon Sungmo, Song Jihyuk and Park Geonil.  Its six members specialized in the areas of singing, dance, acting, rap and pop.  The group official debuted with performed HIT on KBS Musik Bank on September 21, 2007.

Kim Ryeowook: Kim Ryeo-wook (born June 21, 1987), better known by the mononym Ryeowook, is a South Korean singer and actor.  He is a member of the South Korean boy band Super Junior and its subgroups, Super Junior-K.R.Y. and Super Junior-M.  Along with four other Super Junior members, he is one of the first Korean artists to appear on Chinese postage stamps.  He made his solo debut and released first solo album "The Little Prince" in 2016.  He released his duet Cosmic with Bada in 23 September 2016 before he went for enlistment in 11 October 2016 .

Kang Sung-hoon (singer): Kang Sung-hoon (hangul:강성훈, born February 22, 1980) is a South Korean singer from the 90's South Korean boy group Sechs Kies.  Sung-hoon is the main vocalist of SechsKies and is referred to having the best singing voice of the group.  After Sechskies had disbanded in 2000, Sung-hoon pursued a solo singing career, signed under Laful Entertainment at the time.  His nickname Hoony is the title for his third album and his official fan club is called Hoony World.

Kim Ryeowook discography: Kim Ryeo-wook (born June 21, 1987), better known by the mononym Ryeowook, is a South Korean singer and actor.  He is a member of the South Korean boy band Super Junior and its subgroups, Super Junior-K.R.Y. and Super Junior-M.  Along with four other Super Junior members, he is one of the first Korean artists to appear on Chinese postage stamps.  He made his solo debut and released first solo album "The Little Prince" in 2016.  He released his duet Cosmic with Bada in 23 September 2016 before he went for enlistment in 11 October 2016 .

Eun Ji-won: Eun Ji-won (born June 8, 1978) is a South Korean rapper, host, dancer, composer and leader of the first generation idol group SechsKies (젝스키스).  After the group's disbandment in 2000, Eun pursued a solo career in 2001 with his first single ("A-Ha") and has mainly focused on hip-hop ever since.  In addition to his music career, he has appeared on hit shows like "2 Days & 1 Night," "Reply 1997" and "New Journey to the West.

Sechs Kies: SECHSKIES (pronounced as , Korean: 젝스키스 , Japanese: ジェクスキス ) is a six-member South Korean boy band in 1997.  They are the "1st-generation K-Pop idols" and were named as one of the top boy groups in the 90's.  Consisting of members Eun Ji-won, Lee Jai-jin, Kim Jae-duck, Kang Sung-hoon, Ko Ji-yong and Jang Su-won, the group debuted on April 15, 1997.

Lee Jai-jin (musician, born 1979): Lee Jai-jin (Korean:이재진, born July 13, 1979) is a South Korean singer and dancer.  Jai-jin is the sub rapper and main dancer of the South Korean boy group SechsKies, and served as the group's choreographer.  SechsKies disbanded in 2000 but made a long-awaited reunion on 'ToToGa 2' special of 'Infinite Challenge' and signed with YG Entertainment in 2016.

Rap Monster: Kim Nam-joon (Hangul: 김남준 ; born September 12, 1994), better known as Rap Monster, is a South Korean rapper, songwriter, and record producer.  He is the main rapper and songwriter of the South Korean boy group BTS, managed under Big Hit Entertainment.  In 2015, he released his first solo mixtape, "RM".  To date, he has recorded with artists such as Wale, Warren G, Gaeko, Krizz Kaliko, MFBTY, and Primary.

T.O.P (rapper): Choi Seung-hyun (; born November 4, 1987), better known by his stage name T.O.P, is a South Korean rapper, singer, songwriter, record producer and actor.  T.O.P rose to fame in the mid-2000s as one of two rappers in the South Korean boy band Big Bang.  Released under YG Entertainment, the group became one of the best-selling groups of all-time in Asia and one of the best-selling boy bands in the world.  In 2010, while the group was on hiatus, T.O.P and G-Dragon formed a duo to record and release the number-one collaboration album, "GD & TOP".  As a solo rapper, he has released two digital singles, "Turn It Up" (2010) and "Doom Dada" (2013), which peaked at number two and four, respectively, on the Gaon Digital Chart.

Supporting paragraphs: Sechs Kies Eun Ji-won
Answer: Eun Ji-won