Ravio is a playful masked merchant who takes up residence in Link's house in The Legend of Zelda: A Link Between Worlds. He presents Link with a mysterious bracelet that allows Link to merge into walls, and rents out and sells equipment to Link for his use.
At the end of the game, it is revealed that he is actually Link's Lorule counterpart. He served Princess Hilda much as Link serves Zelda, but he was too afraid to stand against Hilda and Yuga when they plotted to steal Hyrule's Triforce. Ravio ran away from Lorule to Hyrule to seek out a hero who could defeat Hilda and Yuga, and happened upon his much more courageous Hyrule counterpart, Link.
In the end, Ravio convinces Hilda that stealing Hyrule's Triforce won't do anyone any good, and remains with her as she sends Link and Zelda back home. He and Hilda are both present in Lorule's Sacred Realm when, by the power of Zelda and Link making a wish on Hyrule's Triforce, Lorule's Triforce is reborn, restoring the crumbling land to life.
When playing A Link Between Worlds in Hero Mode, Ravio's diary can be found in the vacant house in Lorule. Three days of entries are written within and give a brief explanation of Ravio's mindset prior to his escape to Hyrule.