She is 23 years old officially.
Blue Mary has appeared in the following King Of Fighters games:
KOF 97, KOF 98, KOF 99, KOF 00, KOF 01, KOF 02, KOF 03, KOF XI
Blue Mary is the second most popular female character in the Fatal Fury series, falling just behind Mai Shiranui. In her original appearance in Fatal Fury 3: Road To The Final Victory, she fights wearing her leather jacket. However, in the KOF series, she only wears it before the match begins. She takes it off and gives it to her dog, Anton, to keep it safe.
Mary did not appear until KOF 97. SNK decided to allow fans to vote what characters they wanted to see in KOF 97 to make the '97 Special Team. There were 3 polls in 3 different magazines and that was how Blue Mary, aswell as Ryuji Yamazaki and Billy Kane, appeared in the KOF series. Apparently, Mary just barely beat Duck King (from Fatal Fury) by a few mere votes.
Although apparently not appearing in KOF XIII, she is one of the most anticipated to appear in a future KOF title.
As well as being a recurring character in the Fatal Fury series (appearing in every installment since Fatal Fury 3, including a cameo in the Garou: Mark Of The Wolves featurette - Memories Of Stray Wolves), Mary has appeared in :
SNK Vs. Capcom: Card Fighters Clash
SNK Vs. Capcom: Card Fighters Clash 2 Expand Edition
She hasn't appeared in any actual fighting game crossovers, as of yet.
In the KOF games, in most of Mary's pre-fight and win animations, her dog Anton appears.
Apparently, Mary's appearance is based upon Android 18 from the manga and anime series, Dragonball Z - the main physical trait being the short, blond hair.
Aswell as her baggy trousers, combat boots and skin-tight red top, Mary wears an oversized belt. People joke (including members of SNK Playmore) that her belt gets longer and looser in every KOF game.
In Fatal Fury 3, while Mary is searching for Jin Chonshu and Jin Chonrei, she meets and befriends Terry, whom she grows rather attached to. However, it is not actually confirmed that she loves/has a crush on Terry. Interestingly though, in Garou: Mark Of The Wolves, Terry completely shun's B. Jenets advances, meaning either one of two things - he genuinely isn't interested in B. Jenet or else he has a partner, which some people speculate to be Mary. However, none of this has actually been officially confirmed by SNK Playmore.