Arthur is a playable character in Fire Emblem Fates on the Conquest and Revelation paths.
Arthur is one of Elise's retainers and a self-styled "Hero of Justice". Arthur's support with Azura reveals that he grew up as a commoner in Nohr to parents who taught him that justice was acting on what his heart told him to do. He met Azura as a child when she ran away from being mistreated in the castle, and subsequently stood up for her and got punished when the guards came to take her back. Since that time, he studied martial arts and decided he wanted to protect people on the streets from injustice.
If he achieves an S-Support, he will have a son named Percy.