Thanks for the notes, Wayne!
I went to open the file again, and Rivet isn't appearing.
The animation data seems to still be in the file, but the character isn't linked any more. (or the past few versions of my file)
Is there an easy way to "re-link" the proxy?
I didn't delete the character file or anything. It might have to do with creating multiple scenes?
edit: I tried this method, and it hasn't worked https://blender.stackexchange.com/questions/96587/how-to-re-link-missing-blender-files
I fixed it, but I can't really explain how I did it.
Using "Report Missing Files" I could see where Blender expected it to be, so I copied the character to that location, and then it didn't work, and then it did. ¯\_(ツ)_/¯