So, a story idea I've had for a long time is set in an apocalypse (cause not explained), and the two main characters are vastly different.
Tyler is 20 years old, he watched his parents killed in horrible fashion, and turned into a hardened, cold, and rugged survivor. He also is highly skilled in fighting and scavenging.
However, Lucy is 7 years old, and suffers from low-functioning autism. This makes her a savant (this is a central part of the story.) Her parents lived as comfortable as they could, and made Lucy's life as comfortable as possible.
Lucy's ability attracts a local warlord's interest, and he orders her captured for her talent. Tyler happened to witness her escape, and had to fight his way out.
So, why would Tyler save Lucy, when she can barely use the restroom right?