"Beyond Birthday (also known as Rue Ryuzaki, B, BB) was the second child taken in at Wammy's House, in an attempt to create a successor for L, and the serial killer behind the Los Angeles: BB Murder Case. Beyond Birthday has never appeared in the manga, anime, or film series to date, but the Los Angeles: BB Murder Case was briefly alluded to in both manga and anime. Beyond Birthday appears only in the novel Death Note: Another Note - The Los Angeles BB Murder Cases."
Got that off wiki. Hope it helps!