Hilary is an Archon serving Charlescomm. During the Battle of the Expository Bridge, she and five others (Brittany, Miley, Paris, Avril, and Lindsay) were dispatched to spy on Haggar's column and to report their findings back to the capital, while Arkendish communications were blacked out.
It was a dangerous assignment, their veil was blown upon the start of Haggar's turn and three of the Archons were croaked. Hilary immediately flew to the ground and surrendered once they were discovered but her partner was not as lucky. She was tortured by Prince Sammy and his retinue, sustaining a broken leg, injured eye, and other sundry harm but did not reveal anything about her mission or Charlie's plans. Fortunately, when it looked like things were about to get very bad for her, Charlie resumed communication (he ended his Caster link with Vanna) and asked Hilary to open a thinkagram so he could "negotiate" with Sammy.
It is likely that Charlie managed to get her released as part of the deal/blackmail arrangement he forced Sammy to agree to, her current assignment and whereabouts are unknown.
Real World References
She is likely named for Hilary Duff, as her spy group consists of archons named after six young female starlets, five of them singers.