Vilthuril is a doe born in Efrafa who is liberated by Bigwig, and becomes Fiver's mate some time after her escape. She is one of the two does that gives birth to litters (the other being Hyzenthlay to Hazel's litter) after the final conflict with Efrafa. She is quite understanding of Fiver's personal burdens. She does not appear in either the film or the TV series.
In Tales on Watership Down, it is stated that she is also a prophet of a sort, and the other rabbits are happy to leave her and Fiver in their mystic world as long as they come out to beat Rabbits like Vervain.
Vilthuril is among the does that escape Efrafa, but she isn't given any screen time, except when she meets Fiver during silflay. They are both sniffing through the ground, before ending up face to face. She would later become his mate off-screen. It is unknown if they ended up having kits.