Dark Witch Thanatos Eros is the main villain in Stardust Witch Meruru, a anime show within the anime Ore no Imouto ga Konna ni Kawaii Wake ga Nai or for short. Kirino Kousaka, the main protagonist of Oreimo, is a fan of Stardust Meruru and its franchises.
Thanatos Eros is the nemesis of Meruru, the hero of Stardust Witch Meruru. She is an extraterrestrial being from the planet, Evil Star. Dark Witches like her needs a vessel to perform dark magic and humans, especially with those with innocent heart, are found to be ideal vessels. This is the very reason why Evil Star is a threat to the Earth. Her planet has been with war with Comet-kun's planet, Pretty Star. Thanatos Eros herself chose a baby as her vessel making her one of the most powerful dark witches if not the most powerful.
In the Oreimo Animated Commentary 7, She was shown as a pervert-hater. Making assumptions that Kyousuke, the main male character of the series, is doing indecent things to her sister, Kirino when she saw the scene where Kyousuke and Kirino stayed in the same hotel room as part of the latter's research for a novel plot.