The Mountain Sage is a yellow Toad and one of the three Chosen Toads in Paper Mario: Color Splash. He first appears in Daffodil Peak, but later moves to The Crimson Tower and stays there for the rest of the game.
The Mountain Sage is found at Daffodil Peak, but not quite at the top of the mountain as Mario and Huey were led to believe. Instead, the Toad in question is the amnesiac gatekeeper of the mountain, whom remembered nothing of his past before taking the job given to him by the park ranger. However, this fact is not made known until Mario first climbs Daffodil Peak and obtains the Yellow Mini Paint Star, after the park ranger and the gatekeeper insist on there being a Mountain Sage and Mario manages to color in the missing color on the gatekeeper's cap, restoring his memory.
After Mario fills in the color, he recalls everything of his past and his identity. He then demonstrates his power as proof: putting a dent in a mountain and it falls down. The Mountain Sage then departs for the Crimson Tower's gate.
Even after gathering the three Chosen Toads together, it turns out the job is not quite done yet. In order to open the gate, Mario needs to hammer them into their keyholes. After Mario does this, the gate opens and it appears that they are gone. However, this is not so; they are still alive and are able to communicate through the small holes on the gate. They then encourage Mario to go after the Red Big Paint Star. They're seen again at the Port Prisma festival after Mario defeats Morton Koopa Jr. and reacquires the red Big Paint Star, and once more after the defeat of Black Bowser at the Port Prisma celebration.