Prophet of Candor
From Halopedia, the Halo wiki
In 2535, before the Battle of Jericho VII, the Prophet of Candor sent a transmission stating the Covenant were to attack Jericho VII, this transmission was intercepted by Kusanagi who managed to notify UNSC forces.
In 2552, during the Fall of Reach, the Prophet of Candor proceeded to order troops to gain navigation data to Earth and destroy the generators powering CASTLE Base. Kusanagi intercepted the transmission and assisted UNSC forces at the base.
Soon after the planet had fallen, the Prophet of Candor ordered his forces to recover a Forerunner artifact near CASTLE Base. Kusanagi was still active and proceeded to ask surviving UNSC forces to stop the Covenant forces from getting access to the artifact.
Wiktionary describes candor as "Sincere and open speech, honesty in expression". In keeping with the ironic naming convention of most Prophets, this would suggest that the Prophet of Candor had a habit of lying.