In MCU, there is no explanation if someone was guiding Soul Stone before Red Skull so we can assume there was no one. Besides this, this stone doesn't require someone guarding it. One has to make a sacrifice to obtain this, i.e. a soul for a soul. Red Skull couldn't do this so he just guides others to how to achieve Soul Stone.
It seems he was cursed with the knowledge about Soul Stone and to know the name of everyone who arrives there.
It is my curse to know all who journey here.
A lifetime ago, I, too, sought the stones. I even held one in my hand. But it cast me out, banished me here. Guiding others to a treasure I cannot possess.