Nathan is a fairly green member of the J-Team, and for somebody standing at 20 years old, he really lacks talent in the wild world.
That is exactly the reason why he's embarked on an adventure, though. He feels the need to grow his self preservation and learn of his tue self and what the world has to offer.
Not much interesting happened during Nathan's childhood in Unova. It wasn't until high school he was out in the forest and was ambushed by a feral Mightyena that nearly tore him into ribbons. By the skin of his teeth and the blade of his birthday swiss knife along with the help of the newfound companion Ralts that helped him out of the aforementioned jam, the two were inseperable. By the time he hit 20, he was driven out of his home, according to his own statement, and to this day he has refused to enlighten on the circumstances.
Sophia The Gardevoir
Sophia was captured, or more precisely found and recovered by Nathan as a Ralts when he was younger. Having escaped a nasty disagreement inside a community of her species, she fled with her mother, but they were forced to part ways, leaving her to navigate the forest alone. She was isolated for the longest time until she came along Nathan on his stroll and saved him from a grizzly dismemberment. Now a Gardevoir, she has vown to make sure to keep harm away from Nathan, his friends and Pokemon. She shows remarkable curiosity toward the human nature and interspecies interactions, inquiring about said things to a fault when she feels like it.
In a way, she is the team mother, placing the well being of others ahead of her own.