He was youthfully scrawny, his skin bronze enough to look Native American, though his tangled red hair and pug nose argued otherwise. His eyes were an odd shade of brown, so light as to be nearly golden.— Harry Dresden on Fitz
Fitz has the rare ability to hear (but not see) ghosts. He's a smart, natural leader, calm and rational under pressure and able to weigh liabilities and to make and enforce decisions in the face of opposition.
He was just under six feet tall, taller than his companions. Fitz was about age seventeen but an experienced-aged much older look in his eyes. He had boxer-like scars around his left eye.
Fitz chose to join with Aristedes when numerous supernatural bad guys threatened anyone without protection. He witnessed Aristedes kill one right in front of him and it melted. Aristedes uses and abuses him so badly that his chances of survival don't look very good.
Because of Fitz's ability to hear Ghosts, and Dresden's natural concern for kids in trouble with no one to help them, Dresden enlists his help in finding his murderer, and promises to help Fitz and his small gang to escape the tyranny of Aristedes that he is caught up in.
In the series
In Ghost Story, Harry Dresden's Ghost and Sir Stuart Winchester follow the gang of kids in their truck after they shot up Karrin Murphy's home. Stuart causes them to crash. With clear thinking, Fitz gathers all their weapons, unloads them and buries them under the snow. Dresden follows them to their hide-out where he learns that they are all under the thumb of "Baldy" (Aristedes) who is furious and abusive to Fitz for ditching the guns. Dresden talks Fitz, who can hear him, out of reacting to Aristedes in a way that would get him killed. Dresden tells him to meet by the guns at eleven, he'll help him.
Later, after meeting by the guns, Dresden's ghost takes him to Nicholas Christian at Ragged Angel Investigations seeking information on the location of a cult gang he may have heard about—Nick calls them the Big Hoods. Then Dresden takes him to Saint Mary of the Angels where Father Forthill tends to his physical needs: food, shower, rest.
He and Waldo Butters find Dresden in his grave in Graceland. Father Forthill is in danger, Aristedes has him. Butters, Daniel Carpenter, Fitz, Bob, and Dresden's ghost go to Aristedes's hide-out, fight him and free the boys and Forthill.
Fitz moment of truth presents itself when he is running away. Dresden stops him and points out that since both Butters and Daniel are wounded, and Father Forthill is close to death and all the kids he's leaving behind will most likely die without him that he is everyone's last chance. And since Aristedes himself is wounded and weakened, that this was his one chance to stand up to Baldy and win. And, if he ran now, he'd be running all of his life. So Fitz went back and stood up to the bully, got the kids to stand behind him against Aristedes and Forthill was rescued and the kids were freed from the tyranny.