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Frill was a hisser (frilled lizard) that was first introduced in Gregor and the Curse of the Warmbloods. The questers first encounter her when Boots throws her ball into the jungle vines and Frill catches and swallows it. It was then revealed that Hamnet was in alliance with the hisser. Hamnet and his son Hazard lived with Frill and were able to communicate. Frill spoke English and they both spoke some Hisser, though Hazard was better at it.
Hissers are peaceful in nature and will only fight if backed into a corner. Hamnet explained to Gregor that Frill would first camoflage or climb to avoid a fight. If that doesn't work, she would use her frill to create an illusion that she was bigger than she was. If she was forced to fight, she would use his claws and tail to defend herself.
Various individuals rode upon Frills' back. Hazard frequently sleeps behind her ruff; Nike rests after she is injured; and Gregor is carried for a while when he was exhausted from dehydration.
Frill was able to run on her back two legs and uses her speed to get away from the hypnotic flowers that guard the entrance to the star shade field. When the cutters attacked, Frill goes into battle with the questers. Unfortunately, the cutters were able to break through their defenses and Frill died in the fight along with her friend Hamnet.