Kain R. Heinlein

Kain is the younger brother of Marie Heinlein, the wife of Geese Howard and mother of Rock Howard. Unlike his sister, he endured a childhood of poverty in the slums of Second South Town. Grant, then known as Abel Cameron, served as both his closest friend and a father figure.

Nineteen years before the events of Garou: Mark of the Wolves, Kain and Abel witnessed a child's brutal murder in the streets. Traumatized and determined to escape the slums, Kain resolved to seize power by any means necessary. At the age of eight, he entered a major street fight, impressing a powerful crime boss named Don Papas. Recognizing his potential, Don Papas took him in, and Kain eventually rose through the ranks to overthrow his benefactor and claim control.

However, despite his newfound power, Kain was unable to protect Marie. He was later told that she had died—allegedly from illness and the sorrow of an unloving marriage.

Sometime after the events of the third King of Fighters tournament, Kain learned of Geese Howard's demise at the hands of Terry Bogard. Disillusioned with the complacency of the masses, Kain viewed struggle and oppression as the true essence of life. Determined to forge a "world of true freedom," he sought to take over Second South Town, declaring its independence from the United States and reshaping it into a brutal proving ground where only the strong would thrive.

At some point, Kain discovered Rock's existence and saw him as an integral piece in realizing his vision. To this end, he invited Rock and Terry to the King of Fighters: Maximum Mayhem tournament, seeking to test their strength and lure Rock into his fold.

His fate after the tournament remains ambiguous, as multiple endings depict different outcomes. In his own ending, he tracks down Second South Town's remaining crime bosses, culminating in Grant taking full control. In Rock’s ending, he successfully convinces the young fighter to join him—for reasons yet to be fully revealed.

(Source: SNK Wiki)