Guill was a Mari black-market dealer who traded in forbidden violent thoughts and provoked B'Elanna Torres to steal hers.
Guill was a Mari who operated an underground trade in violent images, which were illegal in his telepathic society. Customers who craved the thrill of forbidden thoughts paid him to experience them.
In Random Thoughts, Guill conspired to provoke B'Elanna Torres into an angry thought and capture it for his trade, but the image proved too intense to control and triggered a series of violent attacks. Tuvok tracked him down while investigating the case, exposing the black market and clearing Torres just before her mind was probed.