>>12759037It's how effortless it is. Mumei doesn't try to be sexy or cute, and probably isn't aware how she comes off most of the time. By Holo standards, her outfit is fairly conservative and her color palette is more subdued, giving off a more "normal girl" vibe. That makes it feel like on some level you're appreciating these parts of her without her knowledge, and that subtle feeling of guilt amplifies the effect of her charm.