In life, Trevor was a priest of the Holy Light and friend to Leonid Barthalomew. He died and was risen as an undead during the Third War but was freed from the mindless servitude of the Lich King by Sylvanas. In undeath, he still wears priestly robes in almost tolerable condition and refuses to wield the shadow even if the Holy Light abandoned him.
One day, he discovered a human in the Undercity and recognized him as being the famous paladin, Andarin Lightblade. Leading the chance Trevor proposed him a deal: Trevor wouldn't alarm the guards and Andarin would speak on Trevor's behalf with the Argent Dawn as accepting the priest in their ranks. The deal was made and while the paladin kept the guards at bay the undead freed two prisoners, a mage and a young girl that Trevor knew well because he called her "my princess".
Because Trevor called the girl he rescued "my princess", it can be speculated that the imprisoned girl could be Calia Menethil, the princess of Lordaeron.