Percy Tunstell's valet. It's possible that Virgil used to be Percy's bootblack boy, promoted when Percy dismissed his previous valet. Virgil comes aboard The Spotted Custard with his master, and makes friends with the deckhand Spoo.
In the books Edit
Virgil arrived at Lord Akeldama’s at the beginning of Imprudence with Prim. He reported that Percy had disappeared in a huff. For most of the adventure, he is either attending to Percy's needs or hanging about the Spotted Custard with Spoo.