Carl is a strange creature. A gnome bard by trade, he first appeared in a tavern within Greenbriar. You thought little of him, until he also appeared on the train leading to The Burning Sands. It was there he gave Helios a forewarning, right as the train left the tracks and was ambushed by thieves.
A strange Gnome indeed, nothing else is known about him. But it is likely he will appear again.