Nathan Wallace is the father of Shilo Wallace.
In about 2038, Nathan fell in love with Marni (born 2011), Rotti Largo's fiance. Nathan and Marni married, causing Rotti to harbor a grudge so strong that he waits seventeen years to avenge himself, culminating in the murder of Nathan Wallace and his own death from sickness and age.
Before Shilo was born, Nathan's wife, Marni fell ill. Nathan believed he found a cure only to cause her to lose blood rapidly. Later this is told to be caused by Rotti Largo, by sneaking a poison into his lab. Nathan continued to blame himself for Marni's death.
Nathan went to Rotti for help when Shilo was dying (after being retrieved from Marni's dying body). Rotti provided medicine to be used for Shilo's "illness". In return for this medicine, Nathan became a Repo Man, a man sent out to retrieve organs that the owners neglected to pay for. He keeps this secret from Shilo until the end of the movie where he takes off his helmet after taking a hit to the head from Shilo.
Nathan Wallace is killed by Rotti when Shilo refuses to kill him.