Cassandra Cain is one of the characters that seriously got a raw deal after the DC New 52 reboot. She all but disappeared when she was so cool as Batgirl. They even took her ability to avoid gunfire by reading the shooter and gave that to Nightwing, post reboot. I've heard she's recently made a reappearance, but with no clear explanation of her origin.
In many ways, I put a lot more thought into this than one might expect. As I thought about what Cassie's physical training might be like, I felt that she'd be very spartan. She wouldn't go to a gym or have any machines. She'd work out in her home with the bare essentials. She has a heavy, weighted sandbag, which can be used for a wild variety of exercises, some free weight, a yoga mat (the word on the mat is献身 kenshin, "dedication"), a mirror for checking form, and a water bottle. It's all stuff she could toss in the corner
The only little bit of tech is the heart rate monitor on her wrist that is synched up with her phone. The outfit is based on something the woman I follow for my own physical training wears, but I added a loose little Batman top. I think it looks to be something you'd see a woman wear to the gym.
The funny thing about this picture is that one of the hardest parts was drawing what was in the mirror. I actually set up a small mirror leaning on the couch in the living room then set up my DVDs and Blu-rays to recreate the dimensions of the walls for the room.