As the first few episodes of Season 1 reveal, Shaun and Steve grew even closer during their ceaseless battle with their abusive parents. 

Unable to handle the perpetual war-zone inside the family home, the siblings eventually flee from their violent father, Edward (Kirby Morrow/Michael Trucco) and mother, Katherine (Rebecca Husain), and seek refuge at an abandoned building. 

The boys' plan fails eventually. 

As "Point Three Percent" reveals via a series of flashbacks, Steve died during a friendly game of hide and seek.

One day, Steve decided to help Shaun make friends and overcome the difficulties caused by his autism. To do so, he invited over a group of boys to play a game near the abandoned estate. 

The game took a wrong turn in no time. 

At one point, Steve decided to climb up a defunct train carriage. Unable to gauge the distance correctly, the young boy tripped, fell down on the concrete, and died shortly afterward. 

The tragic accident inspired Shaun to become a doctor — so that he could help anybody who finds himself in a similar situation.