Mendicant Bias is a background AI character in the Halo universe. Information regarding it is available through the terminals in Halo 3.
In a way, Mendicant Bias is the whole reason why the conflict between the Covenant and the humans exists. Mendicant Bias was an AI whose purpose was to research the Flood. He was persuaded by a Gravemind to not allow the Forerunners to light the Halos, allowing the Flood to survive. He attempted to do so by accessing the Ark, where the Halos can be activated from, but he was not able to get there due to the Forerunners destroying all the keyships that granted access to the Ark, along with other defensive locations. The Forerunners created another AI, named Offensive Bias, to slow down Mendicant Bias. The Forerunners had to rush their defense plans to make sure Mendicant didn't manage to deactivate the Halos.
Ultimately, the Forerunners' sacrifices to make sure that Mendicant didn't deactivate the Halos led to their ultimate sacrifice.