Maneater Mildred is commonly associated to be a Butcher, much like her Butcher Knife and Sack suggests. Showing many signs of similarity between her and the Butchers in the Depths, along with the fact that her title bears the word "maneater," it is assumed that she uses her knife to cut up the bodies of men prior to eating them (otherwise known as cannibalism). It should also be mentioned that the person that was going to eat Laurentius of the Great Swamp (according to his dialogue when captured in the Depths) could potentially be Maneater Mildred, but this has yet to be confirmed.
Maneater Mildred is an NPC that invades you upon making your way to the bonfire in the depths if you are in a human state, presumably through the use of a Cracked Red-Eye Orb as there is no association between her and the Darkwraiths Covenant. If you defeat Chaos Witch Quelaag without defeating Maneater Mildred, she will no longer appear for the duration of your playthrough. However, if you defeat her prior to entering the boss fog gate and are in a human state, you'll find her summon sign right in front of the boss fog gate in Quelaag's Domain.
NOTE: Mildred has 0 poise and can be staggered with any weapon, or simply your fist.