Flash is a female Fighting Spider who originally belonged to Glock. She is obsessed with fighting and after Glock died, became the pet of Penn.
Cavern of The Fear
Glock bought Flash sometime after he moved into the palace. He used the spider to gamble against Jinks' spider, Fury. In their first confrontation, Flash won, filling Fury with a need for revenge. When Jasmine and Glock disappeared, Lief and Barda used Fury to find them, as Glock had taken Flash with them. As the companions and Glock killed The Fear, Flash and Fury battled again, this time ending in Fury's victory. The companions were forced to take the spiders with them to the isle of Auron, as the Plume Tribe refused to allow them to remain.
The Isle of Illusion
Because she lost their second battle, Flash was filled with a desire for revenge against Fury. Kree used this to his advantage when Lief, Barda, and Fury were captured by giant Grubs on their way to Auron. He let Flash out of her cage, and the spider tore a path through the grubs and the clay cocoon around Barda to reach his rival, which allowed Jasmine, Filli and Kree to rescue Lief.
Flash and Fury were both brought to the Island of Auron, where they witnessed two Arach destroy Penn's boat. The two spiders realised that they had an even greater enemy that they could not defeat alone and put away their differences. Afterwards, they seemed to lose their desire for violence, and now fought only in play.
Penn became quite fond of Flash and Fury once they stopped fighting, so the companions allowed he to keep them when they went to the isle of Keras.