Vaughan is a Fae male and a member of the cadre of warriors that serve Maeve.
Vaughan took the blood oath to Maeve before the events of Heir of Fire.
Heir of Fire
In Heir of Fire, he and the rest of the cadre answer Rowan's plea for help in defending Mistward. He arrives at Mistward in his avian form. Though Celaena clears a path to Mistward's gates through the Valgs' darkness so the cadre can reach the fortress, Vaughan does not pause to look at her or see if she needs help, much to Rowan's ire.
Vaughan appears to not be present when Maeve meets with Aelin in Doranelle as no one explicitly mentions him being there, but it is likely that he was somewhere nearby.
Empire of Storms
Vaughan does not physically make an appearance in Empire of Storms, but Gavriel and Fenrys say that he is also in Erilea to search for Lorcan - he headed North, as he could handle the much rougher terrain more easily because of his secondary avian form.
Kingdom of Ash
Vaughan, again, does not physically make an appearance in Kingdom of Ash. The characters do wonder however what he is up to and whether or not he is staying away on purpose.
Not much is known about Vaughan's appearance, other than that he has dark eyes and dark hair.
His animal form is a massive osprey with great wings