Agnes is the lead maid of Dante Manor, monitoring the activities of Mary, Jovita, Susan and Alastair. She spends all of her time in her study, constantly working on papers and tasking Jovita with bringing her food every so often.
Because of her impulsive and intimidating attitude, she's not very well liked by her subordinates. Alastair brands her as "a ball of fat" while Jovita suggests that one of the reasons why Mary was "so flustered" was due to Agnes' treatment. Lucius himself is no exception from her unpleasant demeanor, which prompts him to note in his diary: "She's the meanest of them all. I will find a way to make her suffer".
She seems to be a compulsive overeater.
She is one of the two victims (the other one being Michael) who is marked with a cardinal sin. In her case, it is gluttony, which you can spot written in blood on the wall of her study when Lucius first sees her in chapter 5.
She is one of the only two victims (the other being Mary) whose death happens off-screen.