Siyoon x Billlie (Kim Siyoon)

Who is Siyoon (Billlie)?

Siyoon is a South Korean rapper, singer, and lyricist under MYSTIC STORY. Born Kim Siyoon (김시윤) on February 16, 2005, in Geumcheon-gu, Seoul, she is a member of the girl group Billlie and one half of its sub-unit Moon Sua X Siyoon. At 21 years old as of 2026, she stands 173 cm (5 ft 8 in) tall with blood type B. Her MBTI personality type is INFP.

Siyoon's entertainment career began remarkably early. In 2009, at age 4, she appeared on the SBS reality show Star King as part of the Baby Wonder Girls, where they performed 'Nobody'. Prior to joining MYSTIC STORY, she attended Def Academy and Millennium Dance Academy. She was featured in a Reuters article about aspiring K-pop trainees in 2015. Additionally, she participated in the talent show K-pop Star 5 and worked as a model for the Japanese magazine Popteen.

She passed her MYSTIC STORY audition during her second year of middle school. Subsequently, she was officially revealed as a trainee under MYSTIC Rookies on February 18, 2020. After approximately 5 years of training, she debuted with Billlie on November 10, 2021, with their first mini album The Billage of Perception: Chapter One. She later debuted as part of the duo Moon Sua X Siyoon on April 7, 2025.

As the tallest member of Billlie, Siyoon holds the position of main rapper and lead dancer within the group. Her pre-debut fandom name was Unicity, and her logo features a chick popping out of an egg.

Siyoon's Career Journey

Early beginnings and training period

Kim Siyoon's entertainment activities expanded throughout her childhood. In 2012, at seven years old, she appeared in Boyfriend's "Love Style" MV teaser while attending DEF Dance Skool. She was subsequently featured in a Reuters article about young K-pop trainees in 2014. By March 2019, she was training at Millennium Dance Academy, a dance school founded by 2NE1's Minzy.

She was accepted as a MYSTIC STORY trainee in April 2019, around the same time as fellow member Haram. Her training period lasted approximately three years. In October 2019, she traveled to Japan with Tsuki to compete in the 3rd season of "Popteen Cover Girl War," placing sixth among the 10 finalists. Following the competition, she accompanied Tsuki as an occasional model for the magazine.

Debut with Billlie in 2021

She was officially announced as the third member of Billlie on September 30, 2021. The group made their official debut on November 10, 2021, with their first mini album The Billage of Perception: Chapter One.

Moon Sua X Siyoon sub-unit debut

On March 19, 2025, MYSTIC STORY announced that Moon Sua and Siyoon would release a unit album in April. The duo teased their sub-unit during Billlie's concert on March 29, 2025, performing a cover of 2NE1's "Please Don't Go". The official announcement came on April 1, confirming their debut as Billlie's first sub-unit. They released the digital single "SNAP" featuring sokodomo on April 7, 2025. The track is a hyper hip hop song with electronic pop elements, with both members contributing to the lyrics.

Siyoon's Profile and Facts

Kim Siyoon's age and birthday

Born under the zodiac sign Aquarius, Kim Siyoon celebrates her birthday annually on February 16. Her Chinese zodiac sign corresponds to the Rooster. The combination of these astrological signs places her birth year in 2005, making her part of Generation Z in the K-pop industry.

Height and physical appearance

Her weight is recorded at 52 kg (114 lbs), though some sources indicate 48 kg. As the tallest member of Billlie, her stature contributes to her stage presence as both main rapper and lead dancer. Her blood type classification is B, a characteristic she shares with several other members of the group.

MBTI and personality traits

The INFP personality type, also known as "The Mediator," reflects characteristics of introversion, intuition, feeling, and perception. This Myers-Briggs classification suggests a creative and idealistic nature, traits that align with her involvement in lyric writing for the Moon Sua X Siyoon sub-unit. Her personal motto, "It will work out/ I can do it," demonstrates a positive and determined mindset. The INFP type typically exhibits strong values, empathy, and artistic sensibilities, qualities evident in her approach to performance and creative contributions within Billlie.

Siyoon's Role in Billlie

Within Billlie's organizational structure, Siyoon holds dual positions as main rapper and lead dancer. The group operates with an unconventional framework that diverges from traditional K-pop hierarchies, notably lacking an official leader designation. Instead, Billlie implements a system where two members share main positions for each role, with six out of seven members holding main positions. This structure allows for distributed responsibilities and collaborative decision-making, though Moon Sua functions as the de facto leader in practical circumstances.

Her designated team role is characterized as positive/calm action, reflecting her contribution to the group's internal dynamics. Siyoon's representative color within Billlie is yellow, paired with the chick emoji 🐣 as her official symbol. These visual identifiers distinguish her within the group's branding system.

Regarding musical preferences, she has expressed that "Everybody's Got a $ecret" stands as her favorite Billlie track. Her artistic influences include IU and BLACKPINK's Lisa, whom she cites as role models. She was a trainee for approximately five years before joining the group, a period that shaped her development as both rapper and dancer. Additionally, she participates in the sub-unit Moon Sua X Siyoon, expanding her artistic expression beyond the full group format.

FAQs

What makes Siyoon unique among Billlie members? Siyoon is the only member of Billlie who wasn't born in the months of January or September. She shares a dormitory room with Tsuki, Sheon, and Haruna. The hanja characters in her name are 詩 (poetry or verse) and 允 (just or fair).

What are Siyoon's personal interests and preferences? Her hobby includes writing a diary in Japanese, and she enjoys Japanese conversations with Haruna. Yellow is her favorite color, while her preferred food is chicken and her favorite ice cream flavor is cotton candy. She enjoys badminton as a sport.

What unique skills does Siyoon possess? She practiced imitating crow sounds to communicate with the birds that were prevalent in her childhood neighborhood. Many netizens observe resemblance between Siyoon and Weki Meki's Doyeon. She is characterized as crying the easiest among the members. According to Suhyeon, she would be most likely to survive a zombie apocalypse.

What are some lesser-known facts about Siyoon? Her nickname in her mother's womb was 'star'. Her friendship with cube trainee Yu Gaeul originated during their time at Def. At her MYSTIC Story audition, she sang a Heize song but cannot recall the specific title due to nervousness.