Hong So Jin is a pianist/producer who has worked with many singers such as Leessang, Baek Ji Young, 2AM, Sung Si Kyung, Hyorin, JYJ's Junsu, and more. IU and Taeyeon also seem to be close with the musician. In fact, their strong affection for Hong So Jin was shown via Instagram on Valentine's Day (February 14).
Under an Instagram post by Hong So Jin, IU commented, "Ah... I wish you only do IU's concerts." However, Taeyeon disapproved and responded, "Ji Eun (IU's legal name), I'm sorry but hongttochi is mine..."
Fans were happy to see that IU and Taeyeon (the two most popular idols according to a 2016 Gallup poll) still seem to be close friends.