Shiina is a "self-proclaimed female ninja" who fights with traditional Japanese weapons such as dual-wielding kodachi and shuriken. Shiina also wears the special uniform of the SSS, along with a long black scarf covering the bottom part of her face and her neck. Her skirt is slightly different from the usual, as it has two short slits which is probably to aid her in movement. She usually puts up a tough and quiet front, but she is extremely fond of cute things, especially stuffed animals. She is able to sense when danger is coming. She is very strict in her training and would continue with her training even when she is needed to do something else. Shiina's training mostly consist of balancing objects such as broomsticks, scissors and mops on her fingertip, sometimes balancing one on each of her fingers simultaneously while doing something else. She also usually says the phrase "how shallow-minded" when she remarks on her colleagues' actions.
It is revealed in Chapter 24 in Angel Beats: Heaven's Door that Shiina is from ancient Japan. She said she didn't make up a name for herself, so she went by her callsign C7. Yuri invented the name "Shiina" because of the simalarity of the pronunciation of C7. Her life was to train everyday on how to kill people.