Grea Asgeir from Rise of the Weakest Summoner. She starts out as this eccentric professor at the summoner academy and the only one who gives the titular character a chance, which is fine. What isn't fine is when she devolves into a raging self-proclaimed slut even though she was a virgin before the MC and then tells everyone (including royalty) about how she's riding dragon dick with the most foul-mouth and the most dialog of any character in the harem despite being like the the seventh one added. She then robs the life out of every other character by being so painstakingly over-the-top at all times.

Killed the series for me.