Mortimer joins the Raiders in an effort to obtain gold from trading with both them and the Vault Dwellers. He has great plans for the gold, from re-engineering a new body to force the robots at the Whitespring Resort to show him respect, to giving himself a gold paint job, to purchasing weapons modifications for himself, and even to use it to acquire a fourth arm.
Despite being willing to trade with the Raiders, Mortimer looks down on them. He views the Crater Core as a "hole in the ground" unfit for "a Handy of my stature," and despises the interior decorating therein. He also dislikes Ra-Ra for bothering him.
After the events of Secrets Revealed, the last quest in the main Wastelanders questline, Mortimer will arrive at the Crater Core and become available as a merchant. He sells rare plans to the Vault Dwellers in exchange for gold bullion, with plan rarity dependent on one's reputation with the Raiders.
For his inventory, see "Raiders" and "Both factions" sections here.