Death Note Another Note by Nisio Isin is a sort of prequel to the original Death Note manga. It is narrated by Mello, a boy who grew up in an English orphanage alongside L. Mello ‘wrote’ this novel in hopes that Kira and Near would read it. (Near being another boy he and L grew up with, and the one who discovers Light Yagami as Kira)
The novel follows FBI agent Naomi Misora as she is enlisted by L to try to catch Beyond Birthday, a serial killer on the loose in Los Angeles who leaves miniscule clues at each of his murders. Misora quickly realizes that these clues are like a jumbled up road map, where the roads keep intertwining and crossing and it’s all just confusing.
L realizes that these clues being left behind are meant for him, meant to be a game to test his true knowledge against Beyond Birthday.
Why was BB doing this, you ask?
Mello, Near, L, and Beyond Birthday all grew up together at Wammy’s house in England. The four boys, along with the other inhabitants of Wammy’s, were incredibly smart- the smartest children in the world. And they were being raised to become L.
It’s a title passed down through Wammy’s house, and the four boys were the top contenders for the prize. The L we all know and love won, of course, but BB was not happy, to say the least. He wanted to outsmart L- and be the only person to ever do it. (Before Kira, of course.)
While Naomi is working on this case, she’s approached by a private investigator who is… also working on the case. There’s something off about this detective, though, he’s weird. He sits weird, he looks weird, and he definitely dresses and talks weird.
While this investigator “helps” Naomi throughout the novel, he’s really just unraveling the murders in front of her. Because he is the murderer. He is Beyond Birthday.
And he wouldn’t hesitate to kill himself in order to beat L.