Discovered after the victory in Part 23: I'm a Slave for You, Matheus was an old colleague of Mahin from her thieving days in Ikath. Once freed he happily volunteered to crew on the ship the party was on until they got to Marg.
At the end of Part 29: The Death of Captain Jorthra the party found the ship burning and the slaves being picked up by Margish guards. Jorthra was tied to a chair on the dock and warned the party to "Run!!" speaking to Ezra's mind.
The party ignored him and as they approached Matheus appeared and slit his throat. He then fought the party as the master thief he was nearly killing Mahin. The party killed him and his dying words were for Mahin:
"Ardashir always said you were the pretty one. It was a pleasure to kill him."
In his pocket he had a piece of paper with a reward of 1000gp for Mahin's death.