Zoroark she is disguised as a (insert pokemon of your choice) but the disguised fails during sex.
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Maybe as a lucario ? This two are only a generation different and are very known. Maybe too she have humongous boobs but in her lucario form she reduced a bit of her boobs size because she thinked that would freaken out people and when the illusion fail and that the Mc love her huge boobs she will be even more happy ?