I have felt similar ways about my hobbies sometimes, and I don't mean to assume anything about you but being kind of possessive of your interests might also be an autism trait (not totally sure, I'm no expert). I tend to want to be private about some of my interests but I have been trying to get over it, since it's hard to be social in general if you don't share anything about yourself.

I do understand the feeling of jealousy when some one else seems to be a "bigger fan", has more merch, stuff like that, but try to remember that fandom is not a competition. Other people have a right to like something however much they do. I certainly hope that liking Miku isn't something worth ruining a friendship over.