Mary acts like an adult despite apparent middle-school age. When Masumi and Mary speak, the speech honorifics are not those appropriate for an elder Masumi talking to a younger girl. Instead, they appear to indicate the two are on equal levels, or even reverse with Mary expressing greater seniority.
The secrecy surrounding Mary and Masumi's attempts to get her to trust Conan, whom Masumi knows is a shrunken Shinichi, might be indications that Masumi is trying to get the child help about an APTX 4869 problem. At that point in time Masumi did not know Conan has relations with any of the other Akais, including Shuichi.
Mary appears to be sickly and is often seen coughing. This could be a side effect of the ATPX she may have ingested.
In anime, the voice actor for this character has a deep female voice, which resembles a middle-aged woman.