First of all, I should have Googled Eric Harris, clearly.
Second of all: this situation sounds dangerous and not okay.
Third of all: I don’t think this falls into the category of any kind of Otherkin I’ve ever heard of, let alone am comfortable talking about.
Fourth: I want to emphasise that this person, from accounts I have received, does not sound healthy. They also don’t sound like any Otherkin, fictionkin or otherwise, that I’ve ever met. I can’t tell anyone what to do about them, how to feel about them, or anything else. But I do think that avoiding this person, if at all possible, may be the healthiest solution for you, mystery anon. I think Tumblr has methods for blocking people, and if not, Tumblr Saviour will.
Sorry my first answer was so lacking, I had literally no idea what you were talking about!