Old Man Moon was a skinny hunched shape old man with a dense matting of inked tattoos that covered him from head to toe. He had tiny black eyes. Despite his apparent age, he was strong enough to move Hanu, a large Yakshaka in stone armor, on his back with ease.
When Saeng and Hanu found themselves in the Himatan across the Gangrek Mounts after having tumbled into an underground river, Hanu became comatose due to head injuries. Luckily, Ripan found them and called Old Man Moon, who took Hanu with him to his hut.
Here, Old Man Moon struck a deal with Saeng, having her give him a tattoo in exchange for healing Hanu. During the tattooing process, he revealed that he lived the Moon's life, and it his. He had chosen to tie his own life to the Moon's a long time back and taken it as his patron. But since the Moon stole its light from the Sun, he kept fading, signifying his own physical old age and frailty.
Having talked to him a while, Old Man Moon gave Saeng directions to the Temple of Light she was seeking. Having delivered her service, Saeng passed out due to exhaustion. When she came to, she found Hanu healed, and they prepared to move into Himatan. They came across a tiny house no taller than Saeng's knees that exactly resembled the full-size hut where they had just been - it was clearly Old Man Moon's spirit house.