A farily important Jhereg boss, and Vlad Taltos' direct superior in the Organization.
After Vlad killed Tagichatn, he took a map of Tagichatn's territory to Toronnan and drew an area inside the borders that he thought he could control, and showed it to him.
Toronnan took a chance on Vlad, in spite of the fact that he was an Easterner, and let him run the area, giving control to the other parts of Tagichatn's former area to other Jhereg.
Later, Torronan became involved in a Hishi smuggling deal involving the Left Hand and a Jhereg named Bereth. When Vlad ended up out 800 Imperials as a result of this plan going wrong, he reluctantly gave Vlad the okay to pursue the matter.
Also around this time, Vlad began expanding into the area of one of his neighboring Jhereg bosses, Paquitin. Toronnan, observing the fact that Vlad had not properly asked permission before doing this, and knowing that Vlad's powerful friends would be likely to have him revivified afterward, gave Paquitin permission to have Vlad assassinated. Vlad was indeed revivified by Sethra Lavode, but did get the hint, and left Paquitin alone afterward.
Toronnan later threatened Vlad to back off "or else" when Cawti became involved with Kelly's Movement.
Toronnan reported, ultimately, to The Demon, though it's unclear if there are additional levels in the hierarchy between them.