While never being shown, Mortimer is considered to be the Strategist for the Salamanders, implying that he is rather intelligent. Sakuya has said that Mortimer is a cold-blooded player who likely would not hold up his end of a bargain.
It is believed that Eugene and Mortimer are brothers in real life. Presumably he formed a deal with Sigurd to betray Sakuya and Alicia Rue to the Salamanders, in exchange for being reincarnated as a Salamander. However, Sakuya said that Mortimer would likely not have gone completely through with the plan.