L: (Y/N)
He doesn't like nicknames so he just calls you by your name.

Light: Beautiful
He thinks your the most beautiful person in the world so he calls you like that.

Near: (Y/N)
He isn't used to love and things like that so he just calls you by your name.

Mello: Babe
He wasn't sure with which nickname he should call you so he just took that.

Matt: Hot Stuff
He thinks that your hot so that's his name for you.