I is for Illyria
Illyria appeared in season 5 of Angel, which I never watched (maybe one day), but she was the only character in the whole Buffy-universe who started with an 'I' so she made it in. This description of her has been taken word-for-word from Buffyverse Wikki:
"was one of the “Old Ones”, the powerful pure-breed demons that ruled Earth prior to, and during the rise of, humankind. In 2004 (season 5, Angel), she reincarnated in the body of Winifred Burkle. Upon her predestined resurrection, Illyria immediately went about her plan to unleash her army upon humanity, but these plans were thwarted when she discovered the destruction of her army. Lost and purposeless, and driven by the memories of Winifred, Illyria struggled to find her place in the new world, and ultimately joined Team Angel in their fight."