Character Building Help?

[Belial666]

That's exactly how I envision her; Cute-but still Eldritch Horror.

Initially I thought I had to get Human Guise to do it but since the object is that you have to be clearly unsettling and "other", why does it have to be a gross physical change? Besides, an inhuman black-eyed girl is more unsettling than the average bear.

[CMEast]

Due to your description, I'd say human guise is fine. When she switches in to monster mode, even if she isn't actively using her monster powers, it doesn't matter what form she takes as long as people realise she isn't human. She might look like a pretty girl but if everyone is unnaturally terrified of her then I think that still counts (think uncanny valley - http://tvtropes.org/pmwiki/pmwiki.php/Main/UncannyValley)

Perhaps the lighting doesn't fall right on her anymore or it's suddenly hard to focus on her, maybe all the hair stands up on the back of your neck or you feel nauseous or headachy. Maybe you catch glimpses of something horrific in her reflection, or her shadow moves independently. Perhaps she only looks normal when looking directly at her but glance away from her and she'll seem alien in movement.