What anime is this sorry for no image

Iirc the first episode or two has a normal male MC who means some kind of female spirit medium who first with a sword she pulls from her chest. Somehow the guy either dies or get attacked and ends up like her. At some point early on his Sister is attack by a teacher from their school who is secretly some kind of snake monster the two characters eventually kill. The last vague detail I think of is there's some female antagonist with corkscrew pigtails that at some point early on ends up transferring to the same school as the MC.

I have little hope that what can barely constitute as details be enough to get an answer, I mean I could be mixing up details from multiple shows for all know. Thanks in advance.