The acting sheriff of Armadillo, Palmer assumed the position at some point before 1898.
Events of Red Dead Online
If the player becomes a bounty hunter, they can capture various wanted individuals that appear on the bounty board outside Palmer's office, and bring them in dead or alive. Palmer will pay for the price on them and make a comment depending on whether the criminal is alive or dead.
Events of Red Dead Redemption 2
By 1907, Sheriff Palmer is clearly overwhelmed. Armadillo is constantly under assault from gangs and a recent cholera outbreak has dwindled the population, making it a town of lawlessness.
Upon arriving in Armadillo, the player can come across the sheriff being cornered at his office by some Del Lobo Gang members. If the player intervenes, Palmer will thank them and announce his resignation, leaving his official paperwork inside the office. He then departs in the direction of Blackwater.
If Palmer is not assisted in time and the encounter plays out, he is promptly shot dead by the outlaws, who then leave town. This will result in an honor loss.
—Sheriff Palmer's first words to the Del Lobos during the random encounters
Bargain's a bargain. Put your guns away.
—Sheriff Palmer negotiating with the Del Lobos
You cheating sons of bitches
—Sheriff Palmer assisting the player during the gunfight
Oh my, oh my... If it wasn't for you, sir, If it wasn't for you? How lucky we are... the hero we so required. Armadillo is indebted to you. Though I fear it maybe beyond saving... I don't know what you made of that, but it was a close thing. And in light of that, and the sad demise of this settlement, I am resigning my post. You will see I have packed my valise. But the accoutrements of my profession remain in my desk.
—Sheriff Palmer thanking the player after the Del Lobos have been dealt with
Well, so long. I must see about a train.
—Sheriff Palmer before leaving, after greeted by the player
Look, I should like my funds... gentlemen.
—Sheriff Palmer to the Del Lobos after releasing their comrade
No, we had a bargain.
—Sheriff Palmer's last words if he isn't assisted in time
Red Dead Redemption 2
After the encounter, if followed long enough he will despawn.
Sheriff Palmer or an NPC with his model can also be seen in Tumbleweed upon entering the town for the first time, during a random encounter where he brings a member of the Del Lobos to be executed by Sheriff Sam Freeman.