Chevalier Tialys was a Gallivespian spy. He and his partner, Lady Salmakia, worked for Lord Asriel under Lord Roke, Lord Asriel's Gallivespian spymaster. Tialys flew, like many Gallivespians, on an overlarge dragonfly. He flew on one of the dragonflies bred by his clan, an aggressive red-and-yellow insect that died of starvation on the way out of the world of the dead.
The Chevalier Tialys and his companion the Lady Salmakia spied in two different sections of the Holy Church, and later followed Lyra Silvertongue and Will Parry to the world of the dead. Tialys and Will were often at odds and rarely agreed. When Lyra travelled to the world of the dead, Tialys tried to talk her out of it. However, he failed at this and Lyra, accompanied by William Parry, Salmakia, and even Tialys, went anyway.
Dark-haired Tialys was "strong, capable, ruthless and proud." He wore full, capri-length silver pants and a green shirt. No shoes covered his spurred feet. At eight years old, he was a late middle aged Gallivespian.