L's twin. he's got Shinigami eyes (though the debate continues over HOW he has them) and looks exactly like L. he ran away from the Wammy House and became a murderer. he's mentioned maybe once in the manga's and i don't think he was mentioned at all in the show. L had to solve a case called the BB murders of something like that. BB is short for Beyond Birthday so, basically, he had to put his own twin in jail.
Ok, BB(beyond birthday) was a wammy boy. He was one of L's successors. BB had the eyes(debate about how he got them)and he was really being trained hard at the orphanage. There was so much stress being put on him, he ran away. Somehow he got to Los Angeles where he committed 3 murders. Believe bridesmaid, quarter queen, and backyard bottomslash were the names of his victims. Naomi miser and L were working to try and catch him. Naomi misora found him and stopped him from computing suicide. BB was put in jail but this happened a few years before Kira, so he wasn't killed. Basically BB is still rotting away in jail. He's addicted to aardbei jam and he likes to crawl around on all fours. BB is also a genius. He's an awesomely EPIC smokin hot serial killer. Known to have cannibalistic tendanceys.
BB, of Beyond Birthday, was L's seconde successor . He was called B which stood for BackUp. The first successor, of A, killed themselves, because the pressure became too much. After A killed himself, BB left Wammy's House and became a murderer. He has the Shingami eyes, but he did not have to give up any of his life for them, and he does not have a Death Note. There's a whole book about BB called "Another Note" which teaches us all about BB and his past.
Bond Birthday of BB is not L's twin, he was taken in to wammy's house after another guy called A and A and BB was supposed to be L's successors. Born with shinigami eyes, mello wonders if a shinigami dropped it's eyes instead of a deathnote.He was involved with the LA BB murder cases with naomi Misora, in deathnote Misora tells Light before she dies, she worked with L who was actually BB. BB is a serial killer who wants to challenge L to solve the BB case while acting and looking like L, but where is L at this time? L is actually closing cases in a zoek to find him. He is the character from Deathnote: another novel way before deathnote even starts. Anyway he is found out door misora who puts him in prison, at this time Light kills him door using the deathnote because BB is a criminal. Sound familiar?
Beyond Birthday is L's twin, speculated so because Naomi Misora's beschrijving of him in M's recount of the Wara Ningyo case, also know as the Los Angeles BB Murder cases, is exactly like L. He was responsible for 3 murders, linked together door the victims having alliterative names and Wara Ningyo dolls nailed on the walls. Beyond Birthday was planning to be his own final victim, planning to burn himself to death, but Misora stopped him. I don't know what happened to him after that. He was at Wammy's house, and L knew of him, so that's further proof that he was L's twin, of at least close to him.
Beyond Birthday, of 'B', was, together with 'A' (name unknown), one of the first children who where trained to succeed 'L' at some time in the future, just like Mello, Matt and Near. 'B' is older than those, but somewhat younger than 'L'.
'B' is the only known person who was born with the "Shinigami Eyes". It is unknown how this was even possible. [due to this, he is often portrayed with red eyes, even though Shinigami Eyes cannot be distinguished from normal eyes].
'A' committed suicide for some reason when he was still a teenager. It is often speculated that the pressure of succeeding 'L' was too much for him.
After this, 'B' "snapped" and tried to become the most intelligent criminal ever, door creating a case which 'L' would never be able to solve. -- The details of this are the plot of the novel "Death Note: Another Note".
It is assumed that 'B' died in prison door the hands of Kira. but some doubts about this remain.
He is not 'L's brother, as it is sometimes assumed, nor does he look like him. He is a skilled actor however and very talented with makeup, so he was able to disguise himself as 'L'.