Another mask of Ryu's collection, avian was supposed to send a message to his feiance(Or however u spell it) But was shot down by his arch enemy, ARLOND, avian crashed down right next to ryu, he played the song of healing on his ocarina, and laid avian to rest, but after a while, avian's spirit was happy for his death, and happy for ryu's help and courage, he then decided to help Ryu on his journey, giving him his spirit in this mask.

And thats how ryu got this mask, also the raptor's claw is PLANTED oh his arm, AKA he cant take it off.