It's still my character, out there in the setting doing whatever it's doing, and it is and remains my choice - not yours - as to whether or not it interacts (or tries to interact) with the party at some point in the future. You-as-DM can always ask my permission to have it do something or even to take it over completely, but if I deny that permission the conversation ends right there.
I don't really know why this rankles me, but it does, and I'm going to try to articulate my thoughts here. This may not be explicitly discussed during session zero but to me this is the fundamental premise of the social agreement that makes RPGs what they are (regardless as to whether or not PVP is allowed, evil PCs are allowed, or any other number of questions that may come up).
RPGs are a
collaborative work.
The Dungeon Master prepares the world, and "sets the initial state" of all characters except the PCs (among other initial states set), but once player characters are invited to begin acting upon the world, the world no longer belongs wholly to the DM. The players are active agents whose actions shape the world by forcing the Dungeon Master to adjudicate the way in which the world is changed by/reacts to their actions. Every NPC created by the DM - especially one that interacts directly with the PCs but even some that do not due to ripple effects of the PCs actions - is no longer fully under the DM's control - player actions matter and so to some degree, each NPC "belongs" not to the DM, but to the group as a whole. The DM may be responsible for running the NPC at most times, but every character in the world becomes shared among the DM and the players as soon as session one starts.
In the same way, you as a player prepare a character and "set the initial state" of the character, but once session one starts, your character is now acted upon by the world and by other characters. A player may be responsible for running the PC at most times, but every character in the world becomes shared among the DM and the players as soon as session one starts. When a PC dies mid-session, frequently the player is offered a familiar NPC (hireling, ally, etc.) to control for the remainder of that session. That NPC might have originally been created by the DM, but has become a
shared character over the course of play and the DM cedes control of that character to the player.
What makes your PC so special that you cannot fathom ceding control of a
shared character to the DM? You say "it's still my character" - what makes you solely and specially entitled to "possess" that persona?
I suppose most often I play the part of a DM, and so to hear a player say "you cannot control my character!" rings in my ears as "I don't trust you to control this character in a way that is faithful to the way I imagine him." If this is our first session together, I can absolutely understand that. I cannot fathom getting to, say, a tenth session together where I have not earned that trust - and if I haven't, we probably have had to discuss one or both of us leaving the group.
Or perhaps it's just my thinking that
every character in the game world is theoretically capable of being inhabited by a player; the distinction between "PC" and "NPC" is only "is the real-world person currently inhabiting this character currently acting in the capacity of a Dungeon Master?" But then, perhaps that comes from playing at a number of tables where the role of Dungeon Master rotated weekly and quite often PCs were controlled by different players when the "usual" player took on the role of DM. I don't know that this is a "better" or "more virtuous" way to do it, it's just something I have gotten used to and find your statement a bit odd because I've not felt that kind of visceral "ownership" over characters I've created for as long as I can remember (it is very possible that I felt that way very early in my RPG career; I'm in my fifth(!) decade of RPG'ing now and while I remember highlights of my early RPG experiences, I will not claim to remember everything).
If the change is player-driven for whatever reasons, this is absolute. If however the change is something the player had no control over (e.g. my character picked up an item that forced an immediate alignment change), it's still my character and it's on me to play it to its new alignment, ethos, outlook, or whatever.
Okay, your character picked up an item that forced an immediate alignment change. I feel that some of my players (past and present) would be capable of adequately playing the new alignment, ethos, outlook, etc. - and others of my players absolutely would not be capable of (or comfortable) doing so.
But are there cases in which you would feel the character could be relinquished (perhaps when it is barely recognizable as the character any more)? For example, what if your character is slain by a vampire and rises the next night as a vampire spawn? Perhaps the character died 10 years ago and is raised as a mindless skeleton by an evil cleric? Would you object to either of these? What about failing a Will save against a possession? Or is it your position that your control of
absolutely everything about your character is exclusively your domain?
Where does the line get drawn, I guess? When you put your character into an RPG, you potentially subject them to maiming, disfigurement, curses, possession, and even death! All of these things are changes forced upon the character from the outside, and while you might argue you are capable of effectively role-playing them, what exactly is wrong with, say the DM taking control of a possessed character (since presumably the character is no longer in control of their body, the likely-malevolent entity possessing them is in control).
That, and if I'm still in the party the very fact that suddenly you're playing the character and not me is just a bit of a giveaway to the others that something's gone adrift.
Maybe, maybe not. As I mentioned, in some groups I am in, we have had multiple Dungeon Masters and nearly every player has a chance to inhabit nearly every character at some point (this actually is kind of nice so you're not always pigeonholed into being the tank or the artillery or the support).
Again, I think it comes back to trust. If you are in a one-shot, or the first time at the table with a group, there's not enough time for them to have earned your trust for you to think they can effectively inhabit the character you created. But if this is a group that you've been with for a long time, and you don't think anyone but you is capable of inhabiting that character, that to me is a bit of a giveaway that something's gone adrift - why are you still collaborating with these people if you don't trust their ability to collaborate?
Ugh... I wonder if this post is starting to read as a personal attack, which is
not my intent. Rather, I'm asking you to look at the "my character" mindset ... and ask if you can imagine a collaborative group where on any given day, any member of the group could effectively inhabit every character ("PC" or "NPC")? That doesn't mean they would make
every decision and execute
every detail exactly the same way I would... what it means is they would understand the character and all of their decisions would be true to the "essence" of the character - in the same way we can see a dozen renditions of Hamlet, and even though they are not quite identical, each actor in the title role is inhabiting/portraying a "Hamlet" that is true to the character.
If you can imagine such a group, and it appeals to you, and you're not a part of one, I would encourage you to find such a group. I honestly cannot fathom playing more than a couple of sessions with a group of people if I couldn't trust most (maybe not all) of them to inhabit most characters I create (yes, I know, some people are not comfortable with portraying some sorts of characters - I have friends that don't like to inhabit "evil" characters or "quirky" characters or what have you, but for the most part, I no longer see characters as "extensions of me" or "mine" - I see them as roles that anyone who desires to fill that role should be able to step into). I find a group like that, where occasionally I get to step outside a character I have created so I can "examine them from the outside" gives me much greater depth of understanding of who they really are than always trying to inhabit the character and only ever "seeing the character from the inside."