Niles is a playable character in Fire Emblem Fates on the Conquest and Revelation paths.
During his childhood, his father abandoned his mother and she eventually left Niles behind. Eventually, Niles joined a gang of street thieves and they became his family. During his time, he participated in many heists. One of his heists involved invading Peri's family mansion for a "killer doll" her family was rumored to have. Niles eventually realizes that Peri was the "killer doll" during his supports with her.
In his last heist, the gang decided to raid the Nohrian palace, but were almost instantly caught. Looking to save themselves, the gang used him as a scapegoat. It was Prince Leo who found him and, knowing that he had nothing left, begged him to kill him. However, Leo saw worth in Niles and instantly made him his retainer. Since then, he has sworn loyalty to Leo and has abandoned his criminal ways.
If he achieves an S-Support with a female character, he will have a daughter named Nina. He will also be the father of male Kana if he marries the female Avatar and Shigure if he marries Azura.
Niles' eyepatch apparently covers a completely empty eye socket. The skinship quotes with the Avatar have him explain that he lost his eye as a child, at the hands of another criminal.