The Taiwanese assassin with the codename Glass Heart is one of the best professional killers in the world, she is also a young woman in her teens.
On completing her 50th kill she sees that unknown to her the man’s 6 year old daughter was with him and is the one to discover what has just happened, because of this Glass Heart commits suicide by jumping off a building, her heart pierced by a metal fence.
The organization that runs her however is not though with her yet and steals a heart scheduled for transplant.
Glass Heart spends the next year in a coma in which she dreams both about all the people she has killed as well as images from the life of her heart donor Kaori Makimura of Japan who had died saving the life of a child about to be hit by a truck.
Kaori Makimura was the wife of retired mercenary Ryo “The City Hunter” Saeba, who is sure that somehow, whoever it is that becomes the recipient of the stolen heart will someday make themselves known to him.
Eventually, prodded by her persistent dreams about the life of Kaori, the Glass Heart and the City Sweeper do make contact, with the retired mercenary deciding to adopt her as his daughter, he also helps her to recover he real name of Xiang Ying and work toward finding a normal life in the outside world.
The Organization of course has other ideas about this.
The series takes place in a universe parallel to the world of “City Hunter” another manga by Tsukasa Hojo that introduced Ryo Saeba and Kaori, and which has also been turned into an anime series and a film staring Jackie Chan.
There is also a 50 episode anime based Angel Heart.