As a counterpoint to what TheShepard said, I offer this for consideration.
"Tomboys" typically embraced more masculine clothing and behavior. They still often looked perfectly feminine physically, aside from hair styles that were simple for utility purposes such as the various tails, or shorter cuts.
If I were going to make one in game, I might go with the Amazon version of The Treatment, I would consider making her on the taller side of average or above average in height. Definitely higher physique and reflexes. On the clothing side, I would maybe go with a sports bra and boyshorts for underwear, with something like the Strix Threads, Nemean Threads, or something similar in the armor slot.
Basically, she should look tough rather than creampuff.