Disclaimer: Death Note is not mine
B is for Backup.
Second best, not even that with A around.
Not that he hates A, not yet, but he will soon.
He knows that, but strangely it doesn't bother him as much as it used to.
L is for now.
B is a just in case, maybe a little later.
Later never comes, L doesn't die.
B watches the numbers slowly ticking above L's head. Sometimes they speed up, people can't control their own fate, but somehow when L's go up instead of down for an entire afternoon B isn't surprised. Time goes by and still B can't see his own numbers and L's stop for a frighteningly long time. Perhaps L is immortal, perhaps L is a God.
They are strange thoughts, but B is strange and so is L, B just isn't strange in the right way, in the L way. A's numbers dribble away to almost nothing and B realises he doesn't care when he is standing over A's body.
The Backup isn't needed unless the original goes wrong. B watches, B waits.
Somewhere along the line he convinces himself that L really is immortal, really is a God. He doesn't hate L, but gods draw people to them. His victims were going to die anyway; their death might as well serve a purpose. And finally L comes to him, looking far more like himself than B ever could. And L is radiant and his numbers stay still. B calls Lawliet, let's L, God know that he is Birthday's.
L locks him away, God sends sinners to Hell. They drug him until he is delirious and sees shadows where there are none. It was worth it.