Quoneel is a master assassin who trained Tanith Low.
Quoneel lived in an undisclosed location in the early 1900's. He mentored and trained Tanith Low to be an assassin. Quoneel also provided Tanith with the means to kill her first person, a criminal also named Tanith. He seems to be much more concerned about Tanith Low, as he came to her the day after a certain test was completed.
It is later revealed that he knew Vindick Leather.
Quoneel's magic is currently unknown.