Mary Aki is a doctor and researcher affiliated with the Oni Corporation.
Mary is a fair-skinned woman of above average height with a curvy figure and enormous breasts. Other distinguishing features of hers include a mop of shaggy, mid-length black hair with bushy eyebrows and a pair of rounded lips.
Mary is a friendly and easygoing woman who's not afraid to speak her mind, which can often make her come off as blunt and straightforward, such as when she casually dragged Shiki into her apartment and called him "cutie".
It can also be inferred (given the state of her apartment) that Aki either isn't good at keeping a tidy living area, or she simply doesn't like cleaning; although, given what's been shown of her, it's more then likely the latter.
She's also been shown to have little to no shame in regards to her body nor does she have much of an issue discussing matters like sex with others; although, given how she didn't seem to understand Shiki's reaction upon asking the young Oni whether he'd like to have sex with her following the latter's blood extraction, it's possible that Mary could genuinely be unaware of how strange and/or unorthodox her behavior is to those around her.
Content compiled by AnimeList.moe from publicly available sources.
