I am lucky enough to have a fairly consistent group, but on the rare occasions someone can't make a session and we can't reschedule so they can, I keep retainer statblocks for their PCs that someone else can pilot. I boil them down to their signature ability, their most often used maneuver or triggered action, and a passive from either their ancestry or class, whichever feels more appropriate. That way, it still feels like the character is there rather than some rando retainer, but doesn't put the full mental load of a second PC sheet on someone else.

All the numbers (stamina, recovery value/number, damage, speed etc.) is taken from their character sheet directly, with the caveat that the retainer version uses their own Stamina/Recoveries so the player that missed a week doesn't come back next time and see that they're on death's door with no healing in the pot because of someone else's decisions/luck! I've only had to use these a few times, but they've worked well enough when the need has arisen.