Hideki Ryuga (流河 旱樹, Ryūga Hideki) is a Japanese popstar and actor who stars in his own television show, one that seems to be romantic in nature.
Initially, Ryuga is only seen on TV in a show that Sayu Yagami watches, Sayu seems to be a big fan of Ryuga, as shown when she complains at Light who suggested she change the channel while Ryuga was on. Later, when L decides to reveal himself to Light at the To-Oh University entrance ceremony, he uses Ryuga's name as an alias.
L's reason for using Ryuga's name is revealed by Light to not be just an alias, but also a trap, because if Light had risked writing the alias into the Death Note, even as just to see if it was real, he might accidentally picture the real Ryuga's face and kill him instead. Ryuga himself eventually has an actual, albeit brief, appearance in the main story when Misa lands a role as his love interest, presumably in his TV show. While filming a scene where Ryuga is supposed to kiss her, Misa protests, refusing to kiss him due to her uncompromising faithfulness to Light, much to the frustration of the director.
His last appearance was in Pop Music Festival, presented by Kiyomi Takada, he was present among the other artists and guests at the event.