Yumin is a cop in korea, she is a main character in Sun-Ken Rock as well as Sun Ken rock Gaiden. She is the reason why Ken is in Korea.
Ken Kitano's love interest, she left japan to go to Korea and become a cop. It is revealed that she is the daughter of a japanese yakuza boss that killed Ken's family. Yumin hates gangs since her own sister and mother were killed by other yakuza. She has a dragon tattoo on her back that appears when her body temperature rises. Ken's discovery of this tattoo caused Yumin to tell Ken her story from the beginning. Ken would also revealed that her fathers family killed ken's family and she cried in his arms when he told her about it.
Yumin has feelings for ken, for a long time she did not know he was a leader of Sun-Ken Rock until recently when Kim Ban-Phuong. She is very shocked when she learns the truth and thinks Ken was using her like her father does.
Her name is not yumin its yumi and she is japanese despite what she says. It appears she might have romantic feelings for Ken since she wonders if Ken is her "one" and appears to have more and more contact with him as the manga goes on.