I believe the other players, the setting, and the system always have some input on the character that must be acknowledged. A player who wants to begin play with a highly developed character with a detailed backstory, etc., they still have to do so within the restrictions of the system, the setting, and the group. The setting may have restrictions around available types of characters (such as race and class). The scenario may demand that the characters be willing to take on a particular mission. The DM may have certain restrictions like “no evil PCs.” And of course, as RPGs are group activities, your character needs to be able to work with a group (unless PvP is part of the premise, but in that case that’s also a constraint that must be accounted for).
I think you're making a distinction that isn't actually relevant. You seem to be pointing out that the game to be played may create some constraints, along with the players you're playing with. Sure, I can't make Zap Brannagan, clueless womanizer with lightning powers in a grim and gritty game were other players are going to be offended by this character (a very likely outcome, really). So, sure, some constraints, but so long as I'm within the social contract and genre, I have absolute control over my character in my D&D games. The second thing I think you might be referring to are the inputs caused by other players to my decision making process for my character, but this is a second order input at best -- it's the social pressure of the table to how I approach the game in general, not actual asserted control over my character.
So, yeah, in D&D, other players and the game system have, at best, an impact on the player's decision making process and thereby the character, but only through the player's choices. The PC is still under absolute control of the player.
Isn’t that a system question though? For example, in D&D, the player usually has full control over their character’s actions and reactions to events, but certain effects, especially enchantment magic, temporarily subvert that control. On the other hand, some systems, like in the LoU example, dictate or at least influence character actions and reactions to events quite regularly.
I don't think these are at all relevant -- these are all temporary powers, explicitly exceptions, and require no change to the character once completed. They are just transitory effect where the player is largely removed from PC control for a few moments of play before reverting back to the player's control with no lasting changes. The player is entirely in control over how the PC reacts to or even acknowledges the interruption.
I don’t think the player is ever absolute stakeholder of their PC. They are generally majority stakeholder, but various other demands such as setting, system, and group will affect it. The LoU example is a good one of a case where system can affect character.
Unless you're saying the GM is asserting fiat over the rules of the game or that the transitory magics above are being deployed, then no one else ever has the ability to gainsay the control of a PC the player has. You're quibbling around the edges by bringing in peer pressure or manipulation (positive or negative) on the player, which is not the character. You can alter the player, and then the player may make a different choice for the character, but you're not asserting control over that character -- it's control remains entirely with the player.
To revise the example, if I have decided my D&D PC is convinced of their own superiority, then I can maintain this for my PC indefinitely and nothing can ever alter it except briefly with specific magics. There's no one other than me, the owner of the PC, that can choose to have something different occur. I am the absolute stakeholder of this. The best another could do is attempt to change my mind as a player, because they cannot change my character.