I have had a problem with the drumset for my laste two projects (and since my stuff is usually music videos, this is becoming an issue). The drummer "disappears" (see the two examples). They show up in camera view and then disappear when I am putting together the shots.
Just quick guessing...
Too many addons currently in use? If you're trying to render a complex movie with a number of scenes put together in the cutting room, may be MS fails to load in time some of the scene items/characters.
I've had this issue with disappearing props during the rendering process, but haven't seen yet missing characters.
If that's the reason, the only effective workaround implies:
1) enabling only those addons and package that are strictly required to create the movie. Just take a look at the summary info.
2) Split your movie by scenes and/or shots, if needed, and get the pieces together in postproduction, probably with an video edition application.
Besides that, I've seen quite weird effects when enabling the deep of focus with some GPU, including disappearing body parts.