Here's a rabbit hole of a topic again! All the WOBs I'm pulling from will be in a spoiler.
Alright, so, Trell in Mistborn:
In Era 1, Sazed preaches Trelagism to Vin, a religion from an Alaska-like area where the sun doesn't set all the time. This religion, naturally, ends up contained with the Words of Founding, and Miles picks up on it and takes the religion for his own (curiously calling it Trellism, not Trelagism). Trell is further revealed to be another force pressing in on Scadrial, which has been confirmed to be another Shard, one which we had seen at that point. Common theory is that it's Autonomy.
In White Sand, Trell is a foreman. That's all we know about him as far as I am aware.