Thank you everyone for your kind words. I've been avoiding coming back to this thread, as I still get really upset knowing she's gone.
Buddy is lonely. He's becoming so much more of a snuggler in the past few weeks. And he's desperate to be in the bedroom with us at night. Sierra doesn't seem to mind either way, but she avoided Nya in general when she was alive.
I got her ashes back from the vet in a very nice cedar box with her name on it. I've "seen" her a few times around the apartment, out of the corner of my eye. Freaks me out every time it's happened, then I get sad all over again.