Rossart is an unseen character in Game of Thrones. He is dead by the time of the series and only appears in Histories & Lore. He was a Wisdom of the Alchemists' Guild and later served as the last Hand of the King to the Mad King, Aerys II Targaryen.
Wisdom Rossart was a loyal servant of King Aerys II Targaryen, during whose reign the Guild rose again to royal favor. Rossart burned Aerys's enemies alive in the Great Hall of the Red Keep. During Robert's Rebellion, he rose to Hand of the King.
Following the Battle of the Trident, Aerys had Rossart fill King's Landing with secret caches of wildfire to destroy the city and all those in it should the capital fall to the rebellion. However, Rossart was killed by Ser Jaime Lannister during the capital's sacking to prevent him from carrying out the Mad King's order of destroying the entire city.
In the books
In the A Song of Ice and Fire novels, Wisdom Rossart was the head of the Alchemist's Guild during the reign of Mad King Aerys. He had been elevated largely because he shared the king's passion for fire. He was the pyromancer who had cooked Lord Rickard Stark in his armor with wildfire. After Lord Qarlton Chelsted was burned alive under the Mad King's orders, because he refused to take part in Aerys' plan to destroy the city and tried in vain to dissuade him, Rossart became the new Hand of the King, choosing a burning torch as his personal sigil. Rossart served as Hand for less than a fortnight, before being killed during the Sack of King's Landing. Many people who were not in the capital at the time do not even remember that he was briefly Hand of the King.
After being sent by Aerys to kill Tywin, Jaime caught up with Rossart and killed him, then he returned to the throne room and killed Aerys. Before he died, Aerys asked him whose blood was on his sword, to which Jaime replied that it was Rossart's. Jaime noted that Rossart at least tried to make a fight of it, while Aerys did not resist at all when Jaime slit his throat. Days later, Jaime hunted down the other pyromancers and slew them as well. Belis offered him gold, and Garigus wept for mercy, but Jaime killed them anyway.