When Filli was just a baby, he was paralysed by the Wenn as an offering to the Wennbar. Jasmine discovered him and gave him an antidote to the Wenn's venom. Since then, Filli has been one of her closest companions in the Forests of Silence, along with Kree the raven.
He is generally described as a small ball of gray fur. Filli is also known to "chitter" and squeal on some occasions. In Shadowgate, Filli is described as having paws. According to Doran the Dragonlover, Filli is a Siskis. In the anime, Filli is a ball of fur with two eyes.
He generally sits on Jasmine's shoulder, hiding behind her thick, black hair. He rarely makes noise, instead preferring to keep hidden, and is rarely seen throughout the books. He is close with both Jasmine and Kree, and rarely leaves either of them unless told by Jasmine, though unwilling. He also seems to have grown close to Lief throughout the series, as seen in Shadowgate, where he awaited Lief's return to give him an important item. Fili also danced when the belt shined on Lief in Return to Del.
Although small, Filli is able to defend himself with his sharp teeth, such as when he fought the Orchard Keeper. He uses his teeth to bite through a net in City of the Rats, freeing Lief, Barda, and Jasmine from a trap set by Thaegan's children.