A female ninja of the SSS. She likes to keep in the shadows
Shiina plays a largely secondary role in Angel Beats, and no explanation is ever given as to how she died. From what is told in the manga, Shiina was a trained assassin killing anyone without remorse. Originally she was called C7, and had no idea how to name herself, so Yuri opted to call her Shiina from the characters in C7.
It appears that, despite her serious demeanor, Shiina has a weakness for soft and cuddly toys/pets. This is proven when the SSS are travelling to Guild and she sacrifices herself to save a stuffed dog floating away.
She also appears to be strict in her training regimen, seeing Otonashi as somewhat of a rival after he survives longer than her when travelling to Guild.
She will often comment on other people being 'shallow-minded', usually from the shadows, or in the English dub, "this is so stupid."
Shiina is a capable fighter and uses many ninja tools, like shuriken. She also wears a long scarf when outside with exception of episode 4 (the baseball episode), adding to her ninja appearance and some mystery.