>>2233643a huge part of her appeal is exactly because she isn't actually a hololive member. the rarity of her appearances, the fact that she's almost certainly working in the background of any big JP collab, and the general sense of "mystery character", plus the memes due to her being "on a higher level", so to speak, all combine to make her appealing.
also, y'know, the fact that she does have her own model and stuff and occasionally has random unannounced hard-to-catch streams put her into a gray zone between untouchable management and being a character in her own right, and these kinds of gray zones always have their own appeal.