Well, Raubahn has some (massive) story significance beyond being the Grand Flame Marshall, and the Grand Storm Marshal didn't appear until the Leviathan storyline but has since appeared fairly frequently as a side-character, so both of them have had reason to appear in the story. Perhaps they just haven't had a good enough reason to introduce the Grand Serpent Marshall in the Main Scenario yet? As we are heading to Ala Mhigo (most likely) then I could get behind what PArcher said (I never started a Gridanian Adventurer, so I don't know the story enough to confirm that).