Who is Beyond Birthday?~

B.B. appeared in the novel prequel to Death Note, called 'Death Note-Another Note'. The book focuses on the LABB murder cases. A.K.A The Los Angelese Beyond Birthday Murder Cases. He murders three people and then attempts and fails at the last murder (His own suicide) All to create a case L couldn't solve. You see, the Wammy Children had different tactics of taking L's place once (Or before) His reign was up. Beyond simply wanted to surpass him by creating the perfect crime. He led the detective Agent, Naomi Misora (hired by L) through the case, purposly pointing out evidenece to lead up to the ultimate confusion. Sadly for him, L knew all along that B.B. was behind the murders.
Naomi Misora was in the Death Note series for her small part, but the LABB murder cases were mentioned few times, as this video will show~

So, now you know that this prequel is a story all its own for L. B.B may have been crazy, but alot of fangirls adored him (I do too hehe)
Why?
B.B. and L looked ALOT alike. Dressing like L was one confusing motive Beyond used on Naomi Misora, but was there really any reason for him to do so? Maybe he did it to impersonate L more so that if his plans had worked he would become the next L.Even though he dressed in the long-sleeved white shirt just like L, some Fan-made pictures show him in black. I know exactly why.
White is a symbol of purity and good- L.
Black is more portrayed as sin and evil- B.B.
This is known as an archetype, something we all seem to posses is knowledge of symbols and such. I'm not going to explain them to you fully, but in general, black is a bad symbol meaning evil, which is what B.B. is. As L is portrayed as good, wearing white like an angels clothing/robes.
Now, B.B. has special eyes, too. Shinigami eyes. He can see a persons true name and how long they have to live. Although if you own a death note, you can make a deal with a shinigami to recieve it's eyes (giving up half your lifespan for them)for this is useful. Beyond was simply born with them, though. No one knew why. So, why didn't he just kill L using his name? He didn't have a Death Note! Plus, all he wanted to do was create a crime scene L was puzzled on. He wanted to show the world L didn't know everything. He was tryign to humiliate him, in other terms.
So, anyway, what kind of name is Beyond Birthday? Well, with his shinigami eyes he can see 'Beyond' people's 'Birthdays'. Simple, no one knew his real name but maybe himself.

Additional INFO~

Birthday~ Unknown

Death Date~ 1/21/04 (Or /07 in the anime)

Beyond Birthday (also known as Rue Ryuzaki/B, BB) was the 2nd child taken in at Wammy's House, in an attempt to create a successor for L, and was the serial killer behind the Los Angeles BB Murder Case.