This is because I also read Angel Heart by Tsukasa Hojo, the author of City Hunter.
Spoiler
.
.
..
.
Spoiler
.
.
.
.
In Angel Heart, a beautiful but quite ruthless assassin named Xiang Ying was injured(deliberately by herself as she wanted to end her life) and needed a heart transplant. Kaori Makimura recently died and she signed as an organ donor. The female assassin got Kaori's heart after the assasin's group stole Kaori's heart. After the surgery, she went to look for Ryo, the new heart from Kaori influenced her. She became less ruthless and developed kindness. She even somehow developed feelings for Ryo. But Ryo only adopted her as a daughter after finding out the truth.
Xiang Ying because of the heart transplant also inherited the ability to produce a giant hammer out of nowhere to hammer Ryo.
