Riley is a sort of extra Meyerpire that Stephenie added as a plot device in Eclipse. Victoria used him relentlessly and fooled him into thinking she loved him because her Jamesie got horribly murdered by the Cullens, and no female in Twitlight can be taken seriously the slightest without a male mate(and that's totally not sexist at all). At the end of the book he finds out that that wasn't the case (case already stated), but it doesn't matter since he was killed, as punishment for loving Victoria instead of Bella.
Riley was presumably vamped in Seattle. Victoria made him her slave and he controlled the newborn army for her. This was so the newborns, having no idea that Victoria was bossing their boss around, wouldn't think about her, so Edward wouldn't get a mind-tap into them and Captain Compassion and his crew wouldn't figure out who was behind it all. Confused? Don't worry, so are we.
Riley is described as being maybe eighteen or nineteen, with blond hair (so of course he has to die) and red eyes. His only part in Eclipse was to slowly get his fingers and hands torn off by Seth while Eddykins dealt with Tricky Vicky. He is ignored by Victoria through the whole thing. Finally Edward ripped his arms off and Seth finished the job, putting the poor guy out of his misery and out of this series, which is
probably the same thing.