Not much is currently known about Milibeth, however, she, along with the Old Ladies, greets the player upon their arrival to Drangleic.
If the player returns to her after killing the two beach Ogres and the one behind the path by the waterfall in Things Betwixt, she will present the Handmaid's Ladle as a reward.
If the player decides to kill her, she will take out her Handmaid's Ladle as a weapon. Once she is killed, she drops three Human Effigies. After Milibeth dies the Old Firekeepers will no longer have the talk option, thus preventing the player from using the Soul Vessel if they have not already. The only way of reversing this is by absolving sin.