Emily was allegedly the girlfriend of Griffin in Hotel Transylvania 2, although there is some doubt as to her existence. The only time she is seen purportedly holding something (a glass of wine), it was close enough to Griffin's glass that he could've been holding it himself. It is still not clear whether or not she was ever real: Griffin's friends don't seem to think so.
Although Griffin was able to drink wine, when her cup tilted the wine did not disappear but instead fell on the ground, and he called her clumsy.
After holding the seat next to him purportedly because Emily was sitting in it, Griffin also says farewell to her before going on the roadtrip, which is when the wine glass scene happens.
At the end of the film, Griffin bumps into a clearly real invisible woman wearing glasses. She is probably the Crystal who is his girlfriend in the following film Hotel Transylvania 3: Summer Vacation.