We have PCs appear and disappear depending on which players can make it to a given game session. We used to go through all sorts of gyrations to handle this: other players running a character, GM running a character, adjusting the plot to provide reasons for PCs to disappear ("Gandalf went to consult with another member of his order...") Each of these options worked sometimes but not as well at other times.
Now, we just don't waste any time figuring it out. Each player at the table has a PC in play. Simple. If you miss a session, your PC vanishes. If you're back next week, you reappear. To our surprise, the continuity issues haven't interfered with anyone's suspension of disbelief. Indeed, sessions feel tighter and more focused with less meta-discussion of what so-and-so would likely do in any given situation. If the details somehow become important, the GM and player can collaborate on a reasonable explanation that gets spliced into the story.
Now, we just don't waste any time figuring it out. Each player at the table has a PC in play. Simple. If you miss a session, your PC vanishes. If you're back next week, you reappear. To our surprise, the continuity issues haven't interfered with anyone's suspension of disbelief. Indeed, sessions feel tighter and more focused with less meta-discussion of what so-and-so would likely do in any given situation. If the details somehow become important, the GM and player can collaborate on a reasonable explanation that gets spliced into the story.