Someone who goes behind your back and tells your secrets even when you asked them not to.
Then pretend they're innocent and don't apologise in the slightest.
Friend 1: "Helen is such a snake! Can't believe she spilled my secret!"
Friend 2: "Yeah what a bitch!"
Someone who fucks a girl another guy likes. Typically a fuckboy and a backstabber.
Ryan snaked Lauren aka Vince's girl. Vince got angry.
That one fake girl in your class\school who thinks she's better then everyone.