Soukoku no Garou - Hokuto no Ken Rei Gaiden  

Rei Gaiden: Soukoku no Garou, Fist of the North Star: Rei Side Story - The Dark Blue Hungry Wolf, Bloody Wolf's Darkness Blue: Fist of the North Star Rei Side Story, 蒼黒の餓狼 －北斗の拳レイ外伝－, Fist of The North Star Rei Gaiden, 蒼黑餓狼北斗之拳雷伊外傳, 北斗神拳尼尔外传 苍黑之饿狼

Rei was once compassionate but became ruthless due to a traumatic event that took away his humanity. Now, he travels the world seeking revenge and cannot regain his normal human state until his vengeance is fulfilled. As a result, he roams the world as a fierce and solitary predator, hunting down those who stand in his way.