“I'd like to talk about the famous Adventurer's Guild near Pelican Town. The guild leader, Marlon, has a nice rewards program for anyone brave enough to slay monsters in the local caves. Adventurers will receive powerful items in exchange for slaying large quantities of monsters. There's a poster on the wall with more details. Very cool!”
Players meet him the first time they enter the mines. He gifts them a sword, and instructs them to descend into the mines exploring. Players must first complete the quest to kill ten slimes before being allowed admittance to the Adventurer's Guild.
After purchasing the Slime Hutch, Marlon will visit the player, providing them with a Slime Egg to start breeding slimes.
The Adventurer's Guild is open from 2pm-10pm. While it's open, Marlon can be found inside at the counter. He never leaves his spot behind the counter except to go to town Festivals, when the Guild closes entirely.
Marlon lives in the Adventurer's Guild with Gil, who can usually be found rocking in his chair next to the counter Marlon works at.
Players cannot give gifts to raise friendship points with Marlon.
Item Recovery Service
If the player passes out from low health in The Mines, Quarry Mine, Skull Cavern or Volcano Dungeon, Marlon will hold lost items in an "Item Recovery Service" option in the Adventurer's Guild Menu. Players may choose to purchase one lost item, for the same price as it would be sold through the Shipping Box. Items will stay in Marlon's menu until the next time the player passes out from low health, when they will be replaced by any new items lost. Items are not lost if the day ends at 2am; only the gold fee is deducted.
From his dialogue at the Flower Dance, Marlon seems to have a romantic interest in Marnie. Outside of this and his dialogue at the Stardew Valley Fair referencing Clint, there are no other references to his relationships with the villagers.
At the back of the Adventurer's Guild is a bedroom containing a bunk-bed. As they are only two people who are ever seen in the Guild, we can perhaps assume that Marlon and Gil share the bunk-bed.