I was just coming here to pitch Stheno, but I see I've been beaten to that punch. I was just puttering about reading up on Snakes in the wiki but it doesn't seem to say for sure if it's a recently assumed name or if she really was THE Stheno (does seem to hint towards it though). Speaking of, I did notice one thing - when you defeat Snakes, there doesn't seem to be any hospital in their future, Mongoose on Mercy literally gives you a pair of snake skin boots for a souvenir...
I can't recall, when you fight Stheno, do they make it clear or nit if you've outright killed her? if so, she might not count anyway as that wouldn't fit the "still around" bit.