Character agency is a fictional state of affairs - contrast (say) a PC who is charmed with a PC who is not.
Being part of the ficiton, character agency is basically indepenent of player agency.
Some examples form different systems:
* In D&D 4e, a Deathlock Wight has a horrific visage which can make a viewer recoil in terror (mechaincally this is psychic damage, and a push effect with the fear keyword). In recoiling in horror, a character may well be exercising agency (ie there is no need to narrate it as literally involuntary). But the player does not exercise agency at ths particular point of resolution. Once the GM has roled the dice and scored a hit, and the player has not deployed any resources (eg an immediate or free action) to negate the outcome, the player has no choice.
* In Burning Wheel, if a PC fails a Steel check the player gets to choose how to respond: stand and drool, fall prone and beg for mercy, swoon, or run away screaming. This is a reasonable degree of player agency. But in the fictin the response is of course an involuntary one - especially if the character swoons, or stands and drools (ie the character is not exercising agency).
* In my Burning Wheel game, one of the PCs was dominated by a Dark Naga (the spell is called Force of Will; it states that "The caster’s words become thoughts, permanently embedded and resonating against the victim’s personality for the rest of his/her days, as if the victim had formulated them him-/herself; this enables the caster to implant forceful commands into the victim’s mind"). To give this mechical effect, I worked with the player to change one of his PC's Beliefs to reflect this state of affairs - which then gives the player incentives to engage the ficiton having an eye towards that Belief. Following that,the player has played his PC just as normal. Here we see a character with very constrained agency but no particular burden on the player's agency.
* I once played - for a short while - a 2nd ed AD&D game in which the characters had agency (our PCs were awake, undugged, not dominated, etc) but we as players did not: whatever actions we attempted the GM would contrive a reason in the fiction why they didn't work, unless they were the particular action thiat he wanted us to take so as to fit into his predetermined adventure plot. This example shows that there can be character agency without player agency.